The preexistence of the soul refers to the understanding that our souls, as disembodied spirits of some sort, existed before our bodies existed. When this is combined with the understanding that our souls are naturally immortal (and it is almost always so combined), we become possessors of eternal souls. Christianity, of course, confesses God as the only eternal, but I still sense that a lot of Christians today have some belief that their soul somehow existed before they were conceived.
Christianity teaches that we are each created embodied beings and we did not exist before we were conceived. Moreover, in some sense our creation is a synergy between God and our parents. The creation of a whole human being is a beautiful image — much more beautiful to me than the image of shaping a body for an existing spirit to inhabit.
Now, like the immortality of the soul, most people don’t directly talk about the preexistence of the soul. However, I do believe this perception of what it means to be a human being underlies some of the things that are more commonly expressed even among Christians today. I wanted to clearly identify both ideas before proceeding.