The weight of a given shape is shown in a box below that shape. You can change these values yourself by placing your mouse in that box and entering a new weight.
You can drag shapes on and off the trays of the balance. When you do this, a number will appear inside your shape telling you how many objects of that shape you have placed on the tray. The scales will tip to indicate which tray has the larger overall weight. The lower tray is heavier and the higher tray is lighter. If the trays are balanced, each tray holds the same total weight.