real(z) returns the real part of the complex number z. If z is represented as x+iy where x and y are both real, real(z) returns x. Also works on a list of complex numbers.
The real( command is expanded by several assembly libraries (such as XLIB and Omnicalc) to call their own routines. If xLib is installed, then real( will no longer work as intended even in programs that want to use it for its intended purpose.
If you actually want to take the real part of a complex number, and want the program to work with one of these assembly libraries, you could use the Imag( command instead - real(Z) is equivalent to imag(Zi). Alternatively, you could tell people using your program to uninstall xLIB or Omnicalc first.
If a program you downloaded has an error and 2:Goto takes you to a line with real( and a bunch of arguments, this is probably because the program uses Omnicalc or xLIB which you don't have installed.