Atoms are made of three types of subatomic particle: protons, neutrons and electrons. We compare them using relative charge and relative mass, because the real values are far too small to use easily.
A proton has a relative charge of +1 and a relative mass of 1. A neutron has no charge, so its relative charge is 0, and it has a relative mass of 1. An electron has a relative charge of -1, the opposite of a proton, and a relative mass that is very small.
The mass of an electron is about 1/2000 of the mass of a proton, so we usually treat it as almost nothing. Equal numbers of protons and electrons means the charges cancel, so the atom has no overall charge.