You can check out those communities to see more about how they function! Basically you get a randomly generated bingo card, usually 5 x 5 squares, a prompt in each square chosen from pre-announced prompt lists. You can usually exclude certain prompts or categories if you don't want them. And then you try to write stories about the prompts in various squares, enough that the filled prompts form some kind of a bingo line. Different communities/challenges have different types of lines they accept, different deadlines etc. I've never successfully completed a bingo line, but contemplating story scenarios is still fun!

If you want to see more prompt bingo communities, st_aurafina listed a bunch in her comment below!