Exactly what it says. A breed of two different races/species.
Human/drow crossbreeds are quite common and are the results of Drow keeping humans as slaves/lovers.
Another crossbreed is the surface-elf/Drow crossbreed. These are born from technically illegal liasions. But since the surface-elven parent is usually dead by the time the child grows up, nothing can be proved. Nevertheless, it is considered to be a rather distasteful type of crossbreed.
Odder things, like cat/human or lizard/goblin are of course the stuff that a BreedingMistress does.
The "Holy Grail" of every BreedingMistress is the dragon/anythingelse-crossbreed.
It should be possible. Really. If we just could find the right essence-matrix, the proper lifecode and donors for the materials...