Energy is a continues flow, from photons to form, from liquid to gasses, from thought to emotions. When one door closes another one opens, and such is the way of past lives. When we die most spirits are drawn back to earth inorder to incarnate agin and again. The cellular memory defuses back into the earth with our body but the Soul memory remains. This state or memory is often referred to the Akashic Records, a store house of all our souls adventures.
A past life compleation is looking at a former life that holds infomration for you in present time, or a past life the has a strong charge on it and is effecting your current incarnation. Many answers can be found by accessing the wisdom of your souls journey. This is not your first rodeo.