There is a premium tier, called Builders Club, which gives access to premium features, allows you to trade, and removes ads, among other things. To play, you’ll have to jump through some registration and downloading hoops, but the basic game is free.
Admittedly, some fairly decent games also occasionally slip in. As games are built by anyone, really, and quite probably children, they’re full of knock-offs of famous games, brands that have nothing to do with the originals, and weird and inane storylines. You could make - or join - battles, obstacle courses, racing games, treasure hunts, and more. Roblox allows you to create games that are limited really only by your imagination. For the uninitiated, it’s important to remember that this isn’t a game it’s a platform that lets you create games or join games that others have created. Unless you live under a rock (or perhaps far from kids), you’ll probably have heard of Roblox, a user-generated gaming platform open to all but aimed at kids aged 8 - 15.