They are very short stories. About 100 words to maybe 800 with emphasis on the lower end. The contest in this forum is 500 max for example. They're really fun! You can be as absurd as you like because there isn't enough time for suspension of disbelief to kick in. Also it's a nice challenge to try to fit a beginning, middle, and an end into that tiny little space.
Here's an example of a piece of flash fiction in 100 words that I enjoyed. https://www.writingforums.org/threads/new-kid-100-words.151119/
Have fun with the new information. Flash Fiction is actually really fun to write, because it's challenging to write something in such a short amount of space without wanting to go into deeper details. I'm a detail-oriented guy, so they're always something I like to do to change up my writing style. Plus its great practice as well.
Depends who is defining it. There is not one accepted standard. But flash is very short fiction. There are some names for even shorter versions. I once did a 50 word flash fiction story for WD. I think usually they are considered more like 500 words.
Legend has it that there are works of flash fiction as short as 6 words. https://en.wikipedia.org/wiki/For_sale:_baby_shoes,_never_worn
