A person who is given a simple task at his/her job. The most difficult part of the job is comparable to drawing a red X on a sheet of paper using a crayon. A person whose job could easily be given to either a chimpanzee or a preschooler.
“Can you give this work to Mike and make sure he has it done by Friday”
“Mike? He is a total Red X’er, you would be better off giving the work to my pet ferret”