Rotation following player?

So, last year, When I knew nothing about scripting, I joined this game that taught you how to script. It had this big eye. It would look at you. What I mean is that. If you move, the eye rotates to look at you. How did they do that?

Well, I suppose you're trying to find the CFrame that allows one thing to "look" at another. This is easy.

CFrame.new(vector1, vector2)

vector1 is the object's position, and the vector2 is the position that the object (or vector1) will look at.