It is a measure of thermal resistance for some specified thickness of insulation. clo, R value, TOG, and m2 K/W alll measure the same thing and you can convert from one representation to another. clo is the easiest to relate to because 1 clo is the value of a winter weight business suit ensemble that includes a shirt, ties, socks, underwear, and shoes.
For backpackers, clo/oz is very important because it allows us to compare the weight for specified amount of insulation.
This has been covered in the Forums many times. I suggest you do a forum search on this topic for more detail.