They're defined points on a scale -10/10 is foot not touching the floor. 9/10 is just barely touching with the toe, but not really weight bearing. 1/10 is barely perceptible - shortening of stride, slight unlevelness. It's all a bit of an art rather than a science as it's visual perception. So approximately 8-10/10 would be hopping, 5-7 is pretty lame, 3-4 would be lame, 1-2 a bit off.
10/10 tends to be a foot abcess, or a fracture, or a really wussy horse with something else.
Nerve blocks are most effective from about 3 of 10 to 5/6. More than that, or potential fractures would contra-indicate nerve blocks, or at least until x-rays/scans have eliminated fractures/ major tendon damage.
To confuse the issue, there is also a 5 point scale, which does the same (and occasionally gets halfs shoved in). The 10 point scale is theoretically more sensitive than the 5 point, but will often just add to the debate - is it 3 or 4 - either would be a 2 on the 5 point scale!