Atomic mass is how heavy a single atom is. It's measured in atomic mass units (amu).

It’s about the total number of protons and neutrons in the atom.

Detailed (More) Explanation:

Atomic mass is the weighted average mass of all the naturally occurring isotopes of an element, measured in atomic mass units (amu).

1 amu = 1/12 the mass of a carbon-12 atom.

Atomic mass takes into account the different isotopes and their abundance.

It’s close to the mass number (protons + neutrons) but not always exact due to isotope variation.

Example:

Chlorine has two main isotopes: Cl-35 and Cl-37.

Its atomic mass is about 35.45 amu because it's a mix of these two.