If X is an 8-bit value it can't be more than 255 and a check for less than zero is meaningless if it's an unsigned value so I'm guessing it's an 'int' (signed).
It isn't a function in the programming sense, it's a query. It reads as "if the value of X is more than 255 or the value of X is less than zero". It means the following code will only be executed if X isn't between 0 and 255.
As mentioned by others it depends on the size of X. If X is unsigned int its range is 0 to 65535. The Conditional statement check if X > 255 or X < 0. If any of the 2 condition X > 255 or X < 0 is true then the if condition will be true and the if statement will be executed.