secondary succession is when a disaster happens and the only thing left to build off of is soil. you'll see in a sec. you see this forest? let's make something happen, shall we?
wow, looks like that totally natural disturbance has depleted the land of everything excluding its soil. its ecosystem is pretty much gone. there's nothing left!
would'ya look at that? after we time-skipped a few years, stuff is starting to grow back. (if we pretend the trees aren't there because the author couldn't find a more suitable picture...) the grass this one of the first things to grow back, then the tress, and grass dwindles down.
this is called a change in the climax community. one species grows/depletes, an other comes and replaces it. it goes on and on until we get a forest that looks like the pre-disaster one.
we're back and even better! but lets recap. secondary succession is when something horrible happens and the only thing left is soil. using climax community, things go back to the way they were before.