A bog witch is simply a witch whose practices originated or whose main practices center around bogs. A bog is similar in some ways to a swamp. These are areas that are very wet and much of that wetness is saturated with tannins and runoff from the vegetation that is around.
Aside of the unique herbalism and medicine that develops from surviving in such places, they tend to be specialists in seeing what most people would miss in such an environment. There is a lot of fairy lore coming from the various bogs in what is now the U.K.
You may want to check out this blog by a practicing bog witch for more specifics.