What kind of discoloration is it on my laptop?

That's not a stain or anything. It just won't go away.

Wear on the paintwork by the ball of your hand.

The material is worn off.

At that point the laptop got too hot and that burned itself into the material. The best thing to do is to buy a fan for below to put on top. It seems that your laptop can't dissipate the heat enough.

I already had that.