Robux

Robux are probably the least valuable virtual currency in any game ever. Yeah, plenty of games have microtransactions for stupid cosmetic currency. But guess what! Robux isn't JUST cosmetic! You can also buy advantages in almost every Roblox game! Yay!
Despite one Robux being around half a cent, they are actually pretty expensive, because the site doesn't allow you to buy one Robux. The minimum amount of Robux available in microtransactions is 80, which costs a dollar. But 80 Robux can only buy one Adidas hoodie and then you don't have any Robux left for buying Mega VIP on Epic Ultra Obby [*NEW STAGES*] so you need to buy more. The prices rack up eventually, I'm sure.
Nobody wants to spend their birthday money on Robux (unless you're a true Robuxer! #robuxlife #robloxbdayparty #nocentleftunspentonrobux), so lots of sites have been set up advertising "free Robux" to prey on small children who want the Adidas hoodies but don't have any moolah to buy those Robux. These sites do not work. There is no such thing as "free robux". They simply want to take advantage of these kids.
Robux can be spent in just about every Roblox game, whether it be one of the holy premium castes of VIP, Mega VIP, Ultra VIP, VIP+, etc. or on some cheesy broken item in some stupid tycoon somewhere. In obbies, you can buy Stage Skips so that you don't have to play the terrible game. In tycoons, you can buy Infinite Money so that you don't have to play the terrible game. In simulators, you can buy Infinite Stone or Swords or Sand or Dirt or whatever the flip you're mining so that you don't have to play the terrible game.
You can also buy Adidas hoodies, flowery dresses, and cheeky little dragon pets or whatever from the Robux Store. These are actually useless, and just make you look like a buffoon. But I hear it's all the rage within the 10-year-old Roblox gamer community. 
Finally, you can sell Robux back to Roblox for a tenth of their original price as long as you have the enormous scam known as the Builder's Club. So basically any profit you would be making for the Robux goes to your Builder's Club membership anyways.