Total pressure is calculated as the sum or total of all the partial pressures. For example, the air you breathe is made up of a mixture of gases, mainly oxygen and nitrogen. The pressure of air/gas at sea level is 1 bar/ata, and it is made up of 21% oxygen plus 79% nitrogen (78% N2 +1% inert gas). So, at sea level, the pressure of oxygen is 0.21 bar and nitrogen is 0.79 bar, and the total pressure is calculated as 0.21 + 0.79 = 1 bar.
Open circuit works with a fixed amount of oxygen, and the partial pressure changes with the depth. For example, we have 21% oxygen in our air that is affected by the pressure at depth. Partial pressure is important to know on a rebreather or open circuit nitrox dive, which are both covered in additional diver training courses.