Atomic mass is roughly equal to the sum of the individual particle masses of an atom. Atoms have three basic components: protons (positively charged particles), neutrons (non-charged particles), and electrons (negatively charged particles).
Protons and neutrons are the larger particles, and are found in the nucleus, which is the core of the atom. Atomic mass is typically calculated by adding the number of protons and neutrons together, ignoring the electrons because of their small size. Daltons are the standard units used for measuring atomic mass.