Field Strength?

[Gerowen]

What exactly "is" field strength? I've got this little Workman SWR-2T meter that seems to work alright, but it also has an "F.S." bar at the bottom of the meter and is supposed to indicate field strength by just holding the meter in your hand and walking around your antenna while you are holding a dead key. I understand this indicates the strength of your signal at various locations around the antenna, but what do you do with that information after you've got it? Am I supposed to do anything with the knob on the face of the meter while measuring field strength? Just out of curiosity I held my hand-held, 4 watt CB radio and keyed it up with the antenna and jumper cable removed, per the instructions, and the needle never moved in either the FWD or REF positions while turning the knob.