I don't know if the solution to this problem is quite straight-foward but I really don't know how to handle collision resolution on a game where the player walks on an ascendent floor which is not flat. How can the player position itself on the y axis depend on the ground x and z (opengl coords)? What if the floor's slope is too much and the player can't go up, how do you handle that?

I don't need any code, just a simple explanation would be great.