Backchaining is an important concept in training so that you can create complex behaviors or behaviors with multiple parts. Examples could be fetching because there are several parts: go to ball, pick up ball, hold ball in mouth while returning and then dropping ball at owner's feet. Sounds simple, but not all dogs do this naturally. It also is how you can train a dog to put away his toys and many other useful things!
I believe LucyR used backchaining and shaping to create this: