When the osmometer was set up, the solution in the beaker was hypotonic relative to the solution in the dialysis bag, hence, the net movement of water is into the bag. As the water moves into the bag, pressure is generated forcing water up the column. The pressure at any given moment is simply the height of the water in the tube in centimeters. Each centimeter represents 1 gram per square centimeter of pressure. In this case (to the left), the pressure in the bag is 84 grams per square centimeter.
If the tube was long enough, the pressure generated by the rising column of water would eventually stop the net movement of water into the bag. At this point the device is in an equilibrium.
At equilibrium, a concentration gradient still exists across the dialysis tube, but the weight of the water prevents a net movement of water from happening. The solution in the beaker is still hypotonic relative to that in the bag, even though there is no net movement of water.
This is a similar situation to a healthy plant cell where the static pressure exerted by the cell wall against the protoplast stops the flow of water into the protoplast emmersed in a hypotonic medium.