The 100 Coin Stars are stars that Simpleflips always get first, for an unknown reason. but it's probably something really important to the lore why he does this, so i watched all the Super Mario 64 Compilations and i think i found a definitive answer: he doesn't want other people to get the coins, this is proven by how many donations he gets, which means that no other people are supposed to have coins in his world.

100 Coin Stars are unlocked by grabbing 100 coins in a level.