Roblox is a free platform but many games offer in-game (mostly) cosmetic items that can be bought with in-game currency called Robux. There are some games that not only offer cosmetic items but also stuff that can help in being more powerful like better weapons and armor, but you should avoid those because they are basically pay-to-win titles. You can buy Robux for real-world money but purchasing the currency doesn’t change your account type. In order to unlock some additional features, you will have to pick one of the three paid membership tiers.
There are three Roblox paid membership tiers. The first one is called classic and it costs $5.95 a month. Classic tier gives you 15 Robux per day, 100 Robux once you sign for it (it is a one-time bonus) and the ability to join or create up to 10 groups. The next one is Turbo and this one costs $11.95 a month. Turbo comes with 35 Robux per day, a 100 Robux signing bonus, and the ability to join or create up to 20 groups. And the most expensive paid membership is called Outrageous, it gives you 60 Robux per day, it also comes with 100 Robux signing bonus and it gives you the ability to join or create up to 100 groups.
Each paid membership removes ads, gives you access to a virtual hat for your in-game character, gives you access to bonus gear, and allows you to sell stuff and access Roblox trade system.
Now, you can enjoy most Roblox games with a free account. If you occasionally need Robux just make a small purchase so you can access a couple of games that ask for an entrance fee and to get some cool cosmetic items. But if you find free account too limiting for you because you play more games and spend more times on the platform always try with the most affordable monthly subscription because it removes all of the limitations of the free account and it comes with a steady stream of free Robux. Pick one of the two highest tiers only if you really need more Robux or want to create or join lots of groups.