A cation is a positively charged ion. It is formed when an atom or a group of atoms loses one or more electrons. Since electrons carry a negative charge, losing them leaves the atom with more positive charges (from protons) than negative charges, giving it an overall positive charge.
For example, metals like sodium, potassium, calcium, and magnesium commonly form cations. When a sodium atom loses one electron, it becomes a sodium ion with a positive charge. Cations usually combine with negatively charged ions (anions) to form stable compounds such as salts.