It's basically a game with lootbox mechanics. You get opportunities to test your luck and try the probability of getting what you want on them.
But you usually don't get anything good on them because the good stuff has like a .06% probability! It's absurd, but i kinda enjoy it xD
Here's the wiki if I don't explain myself properly x3
https://en.wikipedia.org/wiki/Gacha_game
Ah, so basically "Pay to get stuff, but the stuff you really want is going to need you to buy a lot! No buying, even a snowball has a better chance in Heck!"
Link
JanusMax
Gachas? Uhhh... What's a gacha?