Hi fatalmoon65
I'm not 100% if I understand your question correctly but basically, if you use save, the current game is overwritten and you no longer have access to the old version.
If you use save as, a new save slot is created with the same state your current game is in. So it's basically a exact copy of your save. They are two separate saves and you can play them separately and they end up in different states once you play them.
You can think of it like making a copy of a piece of paper. If you do that, you end up with two exact copies of each other but you can then write something on the copy and it wouldn't have any influence on the original.
Does that answer your question ?