Learn Hardcore German: Die Fußgänger warten am Zebrastreifen, weil die Ampel rot ist. - The pedestrians are waiting at the crosswalk because the light is red.

Because weil introduces a subordinate clause. In subordinate clauses, the conjugated verb goes to the end: ..., weil die Ampel rot ist. In the main clause, the verb is in second position: Die Fußgänger warten ...

Can I use denn instead of weil?

Yes: Die Fußgänger warten am Zebrastreifen, denn die Ampel ist rot.

denn is a coordinating conjunction, so the verb stays in normal main-clause position.
weil is subordinating and pushes the verb to the end.
Nuance: weil is the default for giving reasons in speech; denn is more written/explicit. You can also use da at the start: Da die Ampel rot ist, ... (slightly more formal).

Why is there a comma before weil?

German requires a comma before a subordinate clause. The comma separates the main clause from the weil-clause.

What does am Zebrastreifen mean exactly?

It means at the crosswalk. am is the contraction of an dem. So literally: an dem Zebrastreifen → am Zebrastreifen.

Why is it dative after an?

With location (no movement), an takes the dative: an dem Zebrastreifen → am Zebrastreifen.
If there is movement toward the crosswalk, use the accusative: an den Zebrastreifen gehen.

Why not auf dem Zebrastreifen?

am Zebrastreifen = at/by the crosswalk (typically waiting at the curb).
auf dem Zebrastreifen = on the crosswalk (standing on the stripes).
Both are possible, but they describe slightly different positions.

Why is it die Fußgänger and not der Fußgänger?

Because it’s plural in the nominative case.

Singular: der Fußgänger (one pedestrian)
Plural: die Fußgänger (pedestrians)
Note that the noun’s form is the same in singular and plural; the article shows number.

Is Fußgänger gender-neutral?

Grammatically it’s masculine and can be used generically. To be explicitly inclusive, you’ll see forms like Fußgängerinnen und Fußgänger, Fußgänger*innen, or neutral phrases like zu Fuß Gehende. In Swiss Standard German, you’ll often see Fussgänger (with ss).

Why is rot not rote?

Because rot is a predicate adjective after sein and predicate adjectives don’t take endings: Die Ampel ist rot.
With an attributive adjective before a noun, you do add an ending: eine rote Ampel.

Why is it die Ampel (and not der/dem Ampel)?

Ampel is a feminine noun (die Ampel). In the weil-clause it’s the subject, so it’s nominative: die Ampel.

Can I put the reason first?

Yes: Weil die Ampel rot ist, warten die Fußgänger am Zebrastreifen. The comma stays, and the main clause still has verb-second word order.

What exactly is a Zebrastreifen?

It’s a compound noun: Zebra (zebra) + Streifen (stripe). It refers to a marked pedestrian crossing. A more technical synonym is Fußgängerüberweg. In Switzerland you may also see Fussgängerstreifen.

What’s with ß and ä in the spelling?

ß (Eszett) is pronounced like a strong s (like ss). In Switzerland, it’s usually written ss: Fussgänger.
ä is a fronted a sound, similar to the vowel in English bed but longer/tenser in some contexts. If you can’t type ä, write ae: Fussgaenger (or Fußgaenger).

Shouldn’t it be warten auf something?

Use warten an/am for the location and warten auf (accusative) for what you’re waiting for:

Sie warten am Zebrastreifen auf Grün.
Sie warten auf die grüne Ampel.
Here the sentence only states the place; the reason is given by the weil-clause.

Are there alternative ways to say that the light is red?

Yes:

Standard: Die Ampel ist rot.
Also possible: Die Ampel zeigt Rot.
Colloquial/regional: Die Ampel steht auf Rot. (common, but slightly informal)

Any quick pronunciation tips?

Fußgänger: roughly FOOS-geng-er (the ß is like ss; ä like the vowel in English bed).
Zebrastreifen: roughly TSEH-brah-shtry-fen (the ei is like English eye).
Ampel: AHM-pel.