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.

It's basically a litmus test for whether you have kids or not at this point. The age at which kids become aware of Roblox from their peers is getting younger and younger in my experiences anyway. Before I had kids, I had very little idea either, and I consider myself fairly well acquainted with PC gaming.

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.

And how does it differ from Second Life?