Learn French: Il faut s'arrêter au feu. - You must stop at the traffic light.

Il faut is an impersonal expression, meaning it doesn't refer to a specific person. The il is like the 'it' in 'it is raining'. It literally means 'it is necessary to' or 'one must'. Even though it translates naturally to 'you must' in English, it expresses a general rule here for anyone driving.

Why is there an s' attached to arrêter?

S'arrêter is a reflexive verb, which literally means 'to stop oneself'. In French, you need that s' (short for se) to show that the person is stopping their own motion. Without it, arrêter means to stop something else, like stopping a thief or stopping a machine.

Since il faut is impersonal, how do we know to use s' instead of another pronoun?

Because il faut is stating a general rule here ('one must stop'), we use the default third-person reflexive pronoun se (which shortens to s' before a vowel). If you were saying 'we must stop' specifically, the pronoun would change to match the idea of 'us': il faut nous arrêter.

Doesn't feu mean 'fire'? Why does it translate to 'traffic light' here?

Yes, feu literally means 'fire', but it is also the standard everyday French word for 'traffic light' (which is short for feu de circulation).