Roblox is a game engine and social platform. The basic idea is, you create a game in their engine, using their development tools (such as Roblox Studio), and then you market/promote the game on their website for others to play together with their friends. The game runs on their servers, and you don't have to worry about your own infra if you don't want to, in exchange for the money players spend on Robux (their virtual currency, and now the only one), which they can in turn spend on developer products and other paid items that you set up in your games. Then once you make enough Robux from player purchases, you can cash it out through a process called Developer Exchange which essentially makes you a W-2 employee of Roblox (or whatever financial partner they use, Tipalti?)

The idea is that people like playing with their friends and when they can take their friends and make new friends across hundreds of thousands of games they stay for a long time and (they or their parents) make lots of purchases. The social features of Roblox are a huge part of the appeal, even though I was mostly interested in the engine.