It's exactly what it sounds like, right? At least this is what I have always thought until we actually began the church planting work and started to get this question. So, I will try to dispel as much of my limited knowledge as I can in trying to answer it in a couple of paragraphs.
This may be overly simplistic but when you plant something in the ground (tree, bush, vegetable) you are attempting to replicate something else, another tree, bush, or vegetable. This is what we attempt to do when church planting. We are wanting to replicate what has been happening in church history since Acts chapter 1. We want to replicate a body of believers that is living out the Gospel in their daily lives.
Every church that has ever existed is a church that at some point had to be "planted." That means whether you are sitting in the pew of a 200+ year old church or the folding chairs of a 2 year old church, it was planted by a man, who had a vision, and gathered people to study and live out the Scriptures, thus a church was born. Lord willing the church grows and matures and then, again, becomes a church that takes from itself (money & people) to plant other churches. Just as you use the seeds of other plants and vegetables to plant another plant the same is true for the church. We are planting and producing more churches that reach more people with the Gospel and presenting them mature in Christ. More churches are needed until Jesus comes again, to expand the borders of his Kingdom so we must continue in this hard work of reproducing and replicating ourselves as the church.
Soli Deo Gloria,