As you can guess from the name hildon is the desktop application i.e. the GUI that applications run inside. I'm sure that someone will correct me if I'm wrong but I'd think of it as like gnome or kde an a standard linux box. Not sure about xomap, but since the tablet uses an omap processor I would guess x for omap.
So my guess is that they are the X server and the gui.
Two things you can't really do without.