苑文龙 pretty much answered it. I just want to add that, while 木 today is primarily used to mean "wood," "wooden," or "lumber," its writing does come from the image of a tree, and "tree" is one of its primary meanings. Basically, although these days we call trees 树, 木 still represents tree in characters.
This is similar to the term 口, which represents "mouth" even though the term for mouth today is 嘴巴.