Good answer Jimbo
Good answer Jimbo.
A fixture unit is a fictitious number (from a code which is based on years of collective experience) assigned to a fixture (just a number assigned to a fixture), be it waste, vent, water supply, etc.
It provides a diversity to sizing, in other words if you had two 2" lines from a fixture, you would not size the collector pipe for twice the area of one 2" pipe. This is done because it not highly likely that all the fixtures would be turned on at the same time.
Some codes like the NPC or the IPC have examples on sizing in their appendix.