Они налили вино.

Налили is the past tense plural form of the verb налить, which is a perfective verb.

In this sentence, it means they poured or they poured some.

The perfective aspect suggests a completed action: they finished pouring the wine.

Compare:

• они наливали вино = they were pouring / used to pour wine
• они налили вино = they poured the wine / they poured some wine

In Russian past tense, verbs agree with the subject in gender and number.

The ending -ли shows that the subject is plural.

So:

• налил = he poured
• налила = she poured
• налило = it poured
• налили = they poured

Because они means they, the verb must also be plural: налили.

Here вино is in the accusative case, because it is the direct object of the verb: it is the thing being poured.

But for many inanimate neuter nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: вино
• accusative: вино

That is why the word does not change here.

It can mean any of those, depending on context.

Russian does not have articles like a or the, so вино by itself could be:

• wine
• the wine
• some wine

In English, you choose the best translation from context.
So Они налили вино could be translated as:

• They poured wine.
• They poured the wine.
• They poured some wine.

Yes, Russian often can omit subject pronouns when the meaning is clear from the verb form or context.

So both of these are possible:

• Они налили вино.
• Налили вино.

Including они makes the subject more explicit, and it may add contrast or emphasis:

• Они налили вино, а не мы.
  They poured the wine, not us.

So они is grammatical and natural, but sometimes it is not necessary.

This is a classic Russian aspect pair:

• налить = perfective
• наливать = imperfective

Very roughly:

• налить focuses on the action as completed
• наливать focuses on the action as process, repetition, or background

Examples:

• Они налили вино. = They poured the wine / They poured some wine.
  Completed act.
• Они наливали вино. = They were pouring wine / They used to pour wine.
  Process or repeated action.

This aspect difference is one of the most important things to notice in Russian verbs.

Sometimes, yes.

The verb налить often means to pour (a liquid), but depending on context it can also imply pouring something into a container until it is filled.

For example:

• налить воды в стакан = to pour water into a glass
• налить бокалы вина = to fill the glasses with wine

In Они налили вино, the most basic meaning is simply They poured wine, but context could make it sound closer to They poured out some wine or They served wine.

It does not have to.

This sentence only tells you what they poured: вино.

If you want to say where they poured it, Russian can add that:

• Они налили вино в бокалы. = They poured wine into the glasses.
• Они налили вино в стакан. = They poured wine into the glass.

Without that extra phrase, the sentence is still complete: They poured wine.

A simple pronunciation guide is:

a-NEE na-LEE-li vee-NO

Approximate stress:

• они́
• нали́ли
• вино́

The stress is important in Russian, especially in common verbs like this.

No, Russian word order is fairly flexible.

Они налили вино is a neutral, standard order:

• subject + verb + object

But other orders are also possible, depending on emphasis:

• Вино они налили. = It was the wine that they poured.
• Налили они вино. = less neutral; may sound more emphatic or contextual

For learners, Они налили вино is the safest neutral version.