TL;DR: A weird, blocky video game platform.
This game contains sunscreen eaters, people known as "slenders", and other roblox stereotypes like "ro-gangsters".
Roblox mainly contains lego/brick characters, but may also includes characters called Rthro, which is basically disgusting realistic characters that shouldn't exist on a cartoony website.
Roblox is highly hated by the roblox community for making "terrible" updates such as:
Removing the old death sound, changing the Catalog name to Avatar Shop, and more commonly, banning people for basically no reason.
Game-makers, known as "developers", often put paid items into their games to profit off of children who buy the items. Game-players, just known as players, usually either normally play the game or, like some youtubers or richer players, buy virtual items to have a better experience or ruin other's experiences.
Roblox is or was popularized by a few youtubers, mainly:
DenisDaily, Sketch, Flamingo, EthanGamerTV, MeganPlayz, LankyBox, etc.
The roblox platform includes a currency called "Robux," which must be brought with real money. Robux can offer a few different things, mainly purchasing items to customize your character or to buy in-game items like weapons in some experiences or more in-game money in other experiences.
In general, Roblox is a very weird game platform that can be enjoyed if played correctly, but can also be a terrible, toxic, broken game if played incorrectly.
Bob: Hey, wanna play some Roblox?
Joe: Yeah, sure!