On a plane the normal is the vector perpendicular to the plane. Cause in openGL we send information on the vertex the normal vector is issued as vertex attribute, the normal of the point on the triangle is the (linear) interpolation of the normal on the vertexes.
Normal is usually combined with light direction to shade the surface.
Normal map for example is a technique that use a texture to disturb the surface normal by sampling another vector from a texture.
