In an atom, you have three particles: protons, electrons, and neutrons.
When you add or subtract electrons from an atom, it means that there are either more protons than electrons (positively charged) or there are more electrons than protons (negatively charged).
You cannot add or subtract protons since they are the atomic number, which means the number of protons determines the element. Neutrons have a neutral charge, so they wouldn't affect the charge at all.