A plank is a board or another rectangular piece of wood. When you're building your four-story treehouse, you'll need a lot of planks.
If you've ever watched a movie about pirates, you've heard of "walking the plank," a despicable way to punish captives, forcing them to plunge off the end of a plank (or a "gang plank") into the ocean below. Planks are more commonly used to build houses or hold a fish as it cooks on a grill. Another kind of plank is a political platform, like a candidate's environmental plank, focused on reducing carbon emissions.