What a dobson unit really is?

Question

What a dobson unit really is? In a very literal sense. I know it is .01 mm of the ozone but what does that mean?

Answer 1

It's the height the column would be if all of the ozone in the atmosphere above a square meter of the Earth's surface were concentrated so it was pure gas at 1 atm pressure and 273.15 K.

The analogy is with water vapor, where sometimes you see the water content in the atmosphere given in millimeters.  So if you condensed out all the water, you would get a layer so many millimeters thick on the ground.