Imagine how hard it would be to carry a piece of lumber. Well, Arnold here is making it look easy! But for most of us, hauling lumber would be slow, awkward, and very difficult.
If you think of how you'd move while carrying lumber, that could help you remember that "lumber" also means "to move in a slow, heavy, clumsy way, because you're big and bulky, or because you're carrying something big and bulky."
Here's an enormous alligator lumbering across a path. That means he's moving in a slow way, working hard to move his big, heavy body.
Here's the half-giant Hagrid, lumbering into the shack after breaking down the door. Actually, Hagrid lumbers everywhere he goes: he's almost twice as tall as a human, and not particularly graceful.
As you can see, lumbering is movement that's slow, heavy, and sort of ugly. You might lumber out of bed in the morning if you feel groggy. Or you might lumber around if you're carrying too many heavy things. This poor guy lumbers over to his car, struggling to carry his supplies, then falls over.