Locks vertex normals. Normals have a direct affect on the appearance of shaded polygons. You can modify their appearance by first unlocking, editing the normals, and then locking them.

To change the X, Y, or Z values for normals, you need to unlock the normals using Normals > Unlock Normals. The normal values you specify (or if you use the default values) are fixed for each normal associated with the vertex or vertex/face component. This means that if you change a vertex position, normals do not change position.