The menus aren't aligned right, the changing/fading text needs to be removed/put somewhere else, there is a bug where the info window on top disappears when you select a completely unoccupied slot on the tree, etc. These are the things I can name off the top of my head.
What it's supposed to do? Basically you have a skill tree with dependencies. You can invest points in a first skill at any time (and skill can have more than one point). Certain skills require other skills to already have a certain level before any points in them can be invested. With each level a skill obviously becomes stronger.
That's basically it, it's really no rocket science.