If you look at the normals (on a poly face) it points in the direction of where it faces, so if you have an object say a sphere and you select on of the faces and need to move it out a but but not in XYZ but in the direction its facing you would move it along its normal
normals are perpendicular to the face/edge or even vertex. sometimes its easier to move along the n than take two handles to get the same result. look at the move options (jep, your ordinary move tool) and there is besides local world normal too. click on it and look what happens.
Ah yes silly me, I remembered it during work that the normals isnt the zyx, but Isnt it default to move along normals? (I get it when I hit the move button once, and then again if i want to move xyz axis.)