In the original character of 木 the top is supposed to look like leaves and branches while the bottom looks like the roots. It is used as a radical usually put on the left right or bottom of the character to indicate things associated with the tree.

Radical 75 meaning "tree" is 1 of 34 Kangxi radicals (214 radicals total) composed of four strokes.

In the Chinese Wu Xing ("Five Phases"), 木 represents the element Wood.