What are barrel, muzzle and bore rankings?

I see terms like a TE of 1.5 or an MW of 2.0. Please help me understand these terms and the scale that they use. Still hunting a Garand and am on a slow learning curve. Thanks.

The lower these numbers, the newer your barrel will be. That is, the less wear on it. As pointed out however, this isn't a hard rule for accuracy as one member said he had an accurate barrel that swallowed the gauge at the muzzle.