Learn French: Les voitures arrivant au carrefour s'arrêtent. - The cars arriving at the intersection stop.

It replaces qui arrivent (that arrive / which are arriving). Using the present participle arrivant makes the sentence a bit shorter, which matches how we often say 'the cars arriving' instead of 'the cars that are arriving' in English.

Les voitures is feminine and plural, so why doesn't arrivant have an -es at the end?

When a present participle (ending in -ant) replaces a relative clause and describes an action, it is invariable. This means its spelling never changes to match the gender or number of the noun it belongs to.

Why do the cars s'arrêtent instead of just arrêtent?

The normal verb arrêter means to stop something else (like stopping a machine or a person). Because the cars are coming to a halt themselves, you must use the reflexive verb s'arrêter (literally 'to stop oneself').

Could I say à le carrefour instead of au carrefour?

No, because carrefour (intersection) is a masculine noun. In French, the preposition à (at) and the masculine definite article le (the) must always contract into au. You will never see à le written side by side.