How can I make a smooth level surface?

I am by no means a pro, but I had issues with the floor in my kitchen being out of level when I built my house. Essentially, I think the foundation had a dip in it, and I installed laminate flooring, so every time someone would step there, you could feel the laminate flex a bit. What they ended up doing was pulling the laminate up and "shimming" the floor with some kind of material they spread on the floor and then "graded" to make it flat. I don't know what it was, but it looked like some sort of putty. It goes down wet and dries hard, they filled in the low spot then graded it with a straightedge to make it level with the rest of the floor.