Learn Spanish (Spain): Los chicos están corriendo en el parque.

It comes from the verb correr (to run). To make a word that shows an action happening right now (like an '-ing' word in English), you drop the -er from the end and add -iendo. This gives us corriendo.

Why do we use están instead of son for 'are'?

In Spanish, whenever you are describing an ongoing action with an '-ing' word, you must use the verb estar. Since los chicos is 'they', we use the matching form están. We never use ser to say someone is doing something right now.

Does en mean 'in' or 'at' here?

It can mean either one! The Spanish word en is very flexible and translates to 'in', 'on', or 'at' in English. Saying en el parque is perfectly understood as 'in the park' or 'at the park'.

Could los chicos also mean 'the kids' or 'the children'?

Yes. If there is a mixed group of boys and girls running, Spanish defaults to the masculine plural los chicos. So this sentence could easily mean 'The kids are running' depending on who is actually playing in the park.