Symbols are used to represent numerical values when you have an equation. That way, whenever you see a certain symbol, you can just plug a number into it. Sometimes mass is confused with weight.
Weight is a measure of mass based on how much gravity there is.
Usually it is measured in grams. The symbol for volume, usually measured in Liters. That is all of the pressures added together. It's an important one to remember. Well that T stands for the temperature of the system.