100%x is 100% width of the "parent panel or activity".
100%y is 100% height of the "parent panel or activity".
In you example the myPanel1 is place top and left 0 (upper left corner) and fill the hole width and height of your activity.
If you for example load a layout to a panel with
Panel1.LoadLayout("newlayout")
then the 100%x and 100%y are the width and height of this panel.