This property specifies the position of the top edge of the object offset from the top of it's parent form. If this is a form it's the location of the top side of the form on the desk top. The far top position is 0 in all cases. During the design phase you can generally control the location of an object directly by clicking on it (small control blocks will be displayed around the object), holding the left mouse button depressed and move the object as desired. Once it's located where you want it release the left button and this value will be updated automatically.
You can adjust this property during runtime. It takes any positive integer.