Editing is all about change. Change a word here, a word there, a punctuation mark, a phrase. It's building a better mousetrap. Editing is about amplifying meaning. About bringing the intangible concept closer to a tangible vision scribed on the page.
Nothing compares to analogy to make a concept clear, so here's my analogy of the minute: editing is polishing. Sometimes you start out with a coarse grit, and with each pass use a finer and finer paper; and sometimes all you do is polish with the lightest of cloths. Inevitably the more polish, the brighter it shines. But overworking is just like a sticky wax buildup; and sometimes you have to scrape some away to get to the gleam of what's real.
It's not about quantity; it's about quality; it's not about herding a lot of words, but about finding the right one for the job.
So read on and decide what kind of editing you need. Or maybe you came here because your editor sent you here. In any case, you're welcome to browse.