To wade is to walk through water. If you've been fishing without a boat, you'll have to wade to reach your precious prize (if it gets away, you can brag about how big it was).
When you wade, it doesn't have to literally be through a liquid; wading includes any action that involves a laborious slog through something that offers resistance. For example, you might attempt to wade through a huge book like Moby Dick. And if you want to get into a fight? Wade right on into the brawling crowd. Or if it's verbal fisticuffs you're after, wade right on in to the debate.