And why does it have a number after it?
So, Unreal (as it's also called) is a Gaming Development platform - think of any computer game, and it had to be created somewhere, UE is one of those platforms. There are a few other ones, but for UE is ahead of the game when it comes to using it for film or broadcast use.
The number is just the version - UE4 is widely used, but UE5 has just been launched so you'll see that a lot soon.
Epic Games (the owner of UE)