It's an online store and platform on which people create and play video games. In this sense it is similar to Steam, a storefront/platform for PC games. But unlike Steam, on Roblox all the games are built by Roblox users, many of them children, using a suite of development tools provided by the site.
Game creators make money on Roblox. The site and its games are free to download and play, but users can buy in-game upgrades. They buy and use a virtual scrip called "Robux" to unlock the upgrades. Developers get part of each purchase, in Robux. After developers accumulate a certain amount of Robux, they can convert them into real money.
A publicly traded corporation, Roblox Corp. (NYSE: RBLX) reports that its users have published more than 20 million games. More information is at corp.roblox.com/parents.
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