The treetop is the very top of a tree — the highest branches and leaves. In real use it is nearly always plural: 'the treetops', because you are usually looking across a lot of trees at once, at that uneven green surface you see from a hill, a plane, or a high window. Sun on the treetops, mist in the treetops, birds calling from the treetops. It is a word about looking up, or looking down from above. It also works as a describing word in front of another noun — a treetop walkway, a treetop view, treetop level — which is how tourist places in Taiwan advertise those raised walkways through the forest canopy. One more note: English has a more technical word for the same thing, 'canopy', which scientists and nature programmes prefer when they mean the whole leafy roof of a forest as a system. 'Treetops' is the everyday, more picture-like word. Both are correct; treetops just sounds like something a person saw, and canopy sounds like something a biologist studied.