Robux is the ingame currency of Roblox which is a game made in 2006 where you can buy a lot of things with robux like gamepasses, accesories, clothes, faces and a lot more.
Roblox started being devoloped in 2004 by David Baszucki, Erik Cassel, John Shedletsky and Matt Dusek as Dynablocks but later changed there name to Roblox and at first it did not get very popular but over the years roblox has gotten millions of players and is one of the most popular games on mobile and still very popular on pc too. One of the most popular classic game in roblox is Sword Fight On The Heights IV made by John Shedletsky (a former roblox admin). The most popular game now probably has to be Adopt Me by Dreamcraft.
You can play roblox for free by making an account on: https://www.roblox.com/