A backcross is a cross between a F1 and one of its parents. There are single backcrosses and repeated backcrosses.
A F 2 population, born from a segregation of a F1 holds 3n (n=number of different loci) genotypes. The same F1 backcrossed to one of its homozygous parents holds 2 n genotypes. Hence a backcross decreases drastically the number of genotypes, theoretically facilitating the discovery or detection of the wanted genotype.