Learn Hardcore Portuguese (Portugal): Quando o horário está apertado, como qualquer coisa rápida da lancheira. - When the schedule is tight, I eat something quick from the lunchbox.

horário = your timetable / schedule (work schedule, class schedule, daily routine)
tempo = time in general (time available, or time as a concept), and also weather

So Quando o horário está apertado is literally When the schedule is tight (when my timetable is very full, with little free space).

The definite article o is normal here because we usually talk about o horário as a specific, known thing: your normal schedule. It’s like saying When the schedule is tight in English, not just when schedule is tight. In Portuguese, leaving out the article (Quando horário está apertado) sounds wrong.

What does está apertado mean here? Is it really “tight”? Is this a common expression?

Literally, apertado means tight (like tight clothes, a tight space).

In expressions with time or money, estar apertado means:

to be tight / to be under pressure / to be short

So:

o horário está apertado = my schedule is very full / I’m tight on time
o dinheiro está apertado = money is tight

Yes, this is a very common, natural expression in Portuguese.

In como qualquer coisa rápida da lancheira, what is como? Is it the word for how or as, or is it a verb?

Here como is the 1st person singular of the verb comer (to eat):

eu como = I eat

So como qualquer coisa rápida da lancheira means I eat any quick thing from the lunchbox.

The word como can also mean as / like or how in other contexts (e.g. como se diz...? = how do you say...?), but not here. The position (right after the comma and before a direct object) and the meaning of the sentence make clear that it’s a verb.

Why is the present tense used in quando o horário está apertado, como... instead of a future tense? In English we might say when the schedule is tight, I eat…, but for future events we often think of will.

Portuguese normally uses the present indicative for:

general habits
things that happen regularly in that condition

So Quando o horário está apertado, como qualquer coisa... means:

Whenever the schedule is tight, I eat something quick...
(a general rule / habit)

You wouldn’t normally say Quando o horário estiver apertado, comerei... unless you’re talking about a specific future situation, and it would sound quite formal.

So present + present ( está / como) is the natural pattern for habitual actions.

What exactly does qualquer coisa mean? Is it more like anything, something, or whatever? Is it negative or rude?

Qualquer coisa literally means any thing. Depending on context, it can be:

anything:
- Como qualquer coisa. = I’ll eat anything.
something or other / whatever (a bit vague or indifferent):
- Dá-me qualquer coisa para comer. = Give me something (or other) to eat.

In this sentence:

como qualquer coisa rápida da lancheira
= I eat any quick thing from the lunchbox / whatever quick snack is there.

It isn’t rude by itself. Tone depends on how you say it and the context. Compared with alguma coisa (something), qualquer coisa often feels more indifferent / not very picky.

Why is it coisa rápida (with rápida in the feminine) and not rápido?

In Portuguese, adjectives agree in gender and number with the noun they describe.

coisa is feminine singular.
So the adjective must also be feminine singular: rápida.

Hence:

uma coisa rápida (feminine)
um lanche rápido (masculine)
coisas rápidas (feminine plural)

So qualquer coisa rápida = any quick thing.

Does rápida here mean that the thing itself is fast, or that it is fast to eat/prepare?

Here rápida means quick to eat / quick to grab, or doesn’t take much time.

Native speakers understand:

qualquer coisa rápida = something that is quick in terms of time needed (to prepare or eat), not that the object itself moves fast.

Similar examples:

um jantar rápido = a quick dinner (doesn’t take long)
uma reunião rápida = a short, quick meeting

In da lancheira, what does da mean exactly? Why not just de lancheira or na lancheira?

Da is the contraction of de + a:

de = from / of
a = the (feminine singular)

So:

da lancheira = from the lunchbox

Other combinations would mean something different:

de lancheira (without the article) usually sounds odd here; it would feel like you’re talking about type (e.g. sandwiches of lunchbox-type, which doesn’t make sense).
na lancheira = in the lunchbox (location), not from the lunchbox (source).

Since you’re taking the food from the lunchbox, da lancheira is the correct form.

Why is it da lancheira and not da minha lancheira? Does this change the meaning?

In Portuguese, it’s very common to omit possessives when the owner is obvious from context:

Lavo as mãos. = I wash (my) hands.
Esqueci-me da carteira. = I forgot (my) wallet.

Here, da lancheira is naturally understood as from my lunchbox (or from the lunchbox I usually use). Adding minha:

da minha lancheira

is also correct, and just makes the possession explicit. Both are fine; the version without minha is simply a bit more compact and still clear.

What exactly does lancheira mean in European Portuguese, and are there more common alternatives like marmita or tupperware?

In European Portuguese:

lancheira = a lunchbox / lunch bag, typically for lanches (snacks, light meals), often used by children or workers.
marmita = a food container with a meal, often more like a packed lunch for work; in Portugal this also exists but is often associated with a full cooked meal you bring from home.
tupperware (colloquial, from the brand) = any plastic food container; not necessarily used as a portable lunchbox.

In this sentence, lancheira suggests a portable box or bag where you keep snacks or a light packed meal, which matches the idea of grabbing something quick when time is short.

Could I change the word order and say Quando o horário está apertado, como da lancheira qualquer coisa rápida? Is that natural?

That word order is technically possible and understandable, but it sounds less natural and a bit clumsy.

The most natural orders are:

como qualquer coisa rápida da lancheira
como qualquer coisa da lancheira, rápida de comer (if you really want to separate and stress rápida)

Portuguese tends to keep qualquer coisa + adjective + complement together:

qualquer coisa boa do frigorífico
qualquer coisa quente da cantina

So qualquer coisa rápida da lancheira is the smooth, idiomatic order.