In my critique group, we call them JOPs (short for Junk On Paper). Someone puts out a topic, usually one word, and we give ourselves five minutes to write freely, non-stop, no editing.
Occasionally these little exercises will turn out something worthy of publication, like a short poem. Sometimes they end up being the start of something much bigger. Either way, they are a fun challenge, something to get your writer wheels turning.
Try it yourself and see what good it does for your writer brain!