Dipping is when you take this stuff:
and dip a model in it.
The idea is that you dip the model into the solution, shake it off and then let it dry. The results can be really nice.
This is a marine I base coated in "Codex Grey" also known as "Dawnstone"
It was then dullcoated as the dipped model is very shiney after. For something biological like nids, a shiney sheen might be nice, from what I hear.
If you want to shade a lot of models quickly, dipping is a really quick way of doing it.