Every community has their own unique language that is not familiar to the rest of the world. The same is the Roblox community. Players have slang and a language they use that is only known by those who are on the platform. If you are a new player, you might find these terms hard to master. One of them being IxC.
IXC is a variation of IC which simply stands for In Character. These two terms (In Character) are commonly used in Roleplaying games. If not comfortable with IxC, players also use IsC which also happens to be a variation of In Character.