AVERAGE MASS OF AN ATOM OF AN ELEMENT/ 1/12 THE MASS OF AN ATOM OF 12-C.
This is calculated by multiplying an element's M/Z value by its abundance. This is found out using a mass spectrometer, a topic you will study shortly, so I don't want to confuse you with all the details.
I promise it will make more sense after this topic.
Atomic no. or proton no. which is denoted by the symbol (Z) is the number of protons present in the nucleus of an atom.