Chill out. To relax, especially after energetic dancing or hard work.
Chill (noun) is a feeling of coldness, and chill (verb) is when you put a chocolate mousse you've made in the fridge to make it set.
But chill out (verb) has nothing to do with temperature, it's about relaxing or the dolce far niente after some activity, e.g. lying back in a hammock after a morning's gardening and contemplating the leaves on the trees above.
Chilling out in the summer often involves friends, terraces and cold drinks, like these beers we had on the seafront in L'Escala just watching life go by (sure you'll agree that holidaying is always such hard work).