Learn Swedish: Grabbarna snackar om bilar. - The lads are chatting about cars.

Snackar is a colloquial and informal way to say 'talk' or 'chat'. While you would use pratar (talk or speak) in most everyday situations, snackar fits perfectly here for a casual conversation among friends.

Why use grabbarna instead of killarna or pojkarna?

Pojkarna means 'the boys' (often referring to children), and killarna is a very common word for 'the guys'. Grabbarna specifically translates well to 'the lads' or 'the boys' in a colloquial, buddy-like context.

Why is it just bilar here and not the definite form bilarna?

Just like in English, when you are talking about a topic in general rather than specific items, you use the indefinite plural. If they were discussing specific cars they both already knew about, it would be snackar om bilarna (chatting about the cars).

Is om always the right preposition for talking about something?

Yes, the preposition om is the standard word to use after verbs of communication like snackar, pratar, and talar to indicate the topic of conversation.