A treeless place has no trees in it. Simple — but the word is almost always doing something more than counting. When you say a hillside is treeless, you are usually saying it looks bare, exposed, hard, with nothing to stand under and nothing to break the wind. Writers use it for deserts, high mountain slopes, the far north, wide flat grasslands, and for land where people cut everything down. So it leans towards emptiness rather than towards tidiness. That is why you would not normally call a parking lot or an office 'treeless' — the word wants open land, somewhere trees could have grown. The -less ending is simply 'without', the same one in 'useless' and 'homeless'. It is not the opposite of 'treelike'. And note that it is absolute: treeless means no trees, not 'few trees'. If a slope has a scattering of pines, English says 'almost treeless' or 'nearly treeless', and you will meet those hedged forms very often — writers reach for the hedge because the plain word is so total.