Every game has their money. Usually in a game there are dollars & gems.
In ROBLOX there used to be Tix. There are no more Tix.
Robux is the money on ROBLOX. You can buy: shirts, pants, accessories, gear, game passes, and lots more.
Here is the link to the prices for Robux:
https://web.roblox.com/upgrades/robux
Robux can be earned by group funds and making a game.
Robux can also be made by purchasing a builder's club (BC) (You will learn about soon)
Here is the link to the items you can buy with Robux:
https://web.roblox.com/catalog/?Direction=2&PageHash=7f098b52d3626402ccc8e0091de594dd
You can buy game passes in-games.
And, be careful! Free-Robux does not exist!