Basically, if you die, you restart the map not from the very start, but at some point in the middle of it. I believe it requires SessionVars to work, but what I've tried so far isn't working. I definitely know it's possible, and I can list a few examples of maps that have this system, but I haven't been able to find any forum posts or open source maps that explain this.
Thanks in advance.
EDIT: So I actually managed to fix the problem myself.
Still, feel free to post your method here for the convenience of others who may have this same question.
Later I may edit this message so that it includes my own method, as the checkpoint system is still fairly new to me.