It's just as the name implies, you can check in for your resort reservation online before arrival. I think it opens something like 10 days ahead.
Lots of folks really like it, as it speeds up the check-in process somewhat, but I hate it.
I like to see what room is assigned, and ask for a change if I don't like the location - which, if you have checked in online, means getting in the check-in line anyway. I also like talking to the CMs and don't mind the process. To me the extra 10-20 minutes or so is worth it. Others see this as a waste of time.