In very simple terms, Roblox is an MMO based exclusively around user generated contents (games, items, assets...), including its own virtual currency, microtransactions, marketplaces and convertibility to/from real money. Roblox as a company takes pretty hefty cut from all transactions.

There has been a silent shift in the gaming market over a long time now. Roblox is one aspect of it. Another is the absolutely massive amount of money raked in by some free to play mobile phone titles. For example, Playrix has a revenue comparable to Ubisoft, but their main products are a series of match-3 type games for phones.