There are two different types of normal. Face normal and vertex normal. Face normals point in the direction of the face, and when flipped, the face is only visible from the other side (unless you have "backface culling" turned off - in which case it will still be visible from both sides).

Vertex normals specify how faces are shaded between.

This video might help

https://www.vtutorials.com/maya/normals.wmv