If you are asking about the concept of "normal" I might can explain that a little bit.

Normal basically means perpendicular. So the surface normal is a vector (like a line) that extends from a surface at a 90 degree angle.

Hmm, pictures would help I guess.

Imagine you have a surface representing a horizontal floor. The normal to the floor would be a vertical vector.

^
| <-- This vector is normal to the floor
|
_________ <-- This is suppose to be the floor

Or is you had a vertical wall, then normal to the wall would be a horizontal vector.

| <--This is the wall
|
|-----> This vector is normal to the wall
|
|

of course you can have a normal to any surface at any angle, these are just the simplest examples. Maybe that will help some, I hope.