In cannabis breeding, a backcross is a plant that has been bred with itself or one of its parents; inbreeding, essentially. Backcrossing is done to bring out or strengthen certain genetic traits of a cannabis strain, such as flavors, aromas, effects, high yields, or resistance to mold, and is common in commercial cannabis breeding. 

What is a backcross?

When two cannabis strains are crossed to create a new hybrid strain, high quality breeders will usually cross that new strain again with a genetic clone of itself, or one of its parents, to strengthen some of its traits. This is called backcrossing. 

A strain that you buy at the dispensary has likely gone through months or years of growing, crossing, and backcrossing.