What is being measured by the simple instrument illustrated?

Explanation

If the tube were open at both ends, the level of the liquid would be the same on both sides of the “U” in the tube, since the atmospheric air would be exerting equal pressure on both ends. But the higher level on the right side of the “U” indicates that the pressure from the air tank is greater than the atmospheric pressure. (The various level marks up and down the tube provide a quantitative measure of that pressure.)