Need Help on Nurb surfaces

I have recently just started working with
OpenGL. I am having trouble creating a Nurb
surface with the gluNurbsSurface function.
I just don’t understand the concept of knots.
The manual I am using as reference does not clearly explain the concept of knots and how they are applied to the nurb surface. By looking at other
sources knots are explained as parametric equations that go in the u and v direction.
I am able to create nurb surface but how can I use the control points and the knots which go in both th u and v direction to produce the the surface I want? Can Someone explain me in simple terms what knots are?