Checkpoints. How do they work?

[Pandemoneus]

Create a PlayerStartArea.
Then make a ScriptArea somewhere else that will trigger the checkpoint.
Add a EntityCollideCallback with that script area and put CheckPoint(string& asName,string& asStartPos ,string& asCallback, string &asDeathHintCat, string &asDeathHintEntry); into the function, whereas the StartPos is your new PlayerStartArea.

I know what to fill in at the name and for count it should be. But I don't understand the count. So if a player dies. How does the game know that he has to load at that point?

The count means the amount of deaths the player already had. That function will always be called when the player dies (like in the main story, where monsters spawned at other spots and after some deaths didn't spawn at all anymore).