As is true with almost every label you can find in the literary world, there isn’t much for a consensus on the actual rules for what defines flash fiction, but the very basic concept is that it is a very short form of fiction.
The idea is that it is something that can be read quickly and written relatively quickly, which gets through the basic process of telling a story in very short order. Longer than five words, shorter than 1500.
The most famous piece of flash fiction is Hemingway’s “For sale: baby shoes, never worn.”
One of the greatest things about flash fiction is that it’s perfect for experimental fiction. So, if you have an idea that you want to try out, give it a thousand words and see if you can make it work.