When an atom or molecule gains an electron, it is said to be reduced. In the context of chemistry, reduction refers to the process where a molecule or atom decreases its oxidation state, which often occurs through the addition of electrons.
Reduction is part of a larger reaction type known as redox (reduction-oxidation) reactions. These reactions are characterized by the transfer of electrons between species. When one species gains electrons (is reduced), another species must lose electrons (is oxidized). Therefore, if you find that an atom or molecule has gained an electron, it has undergone reduction and its oxidation state has been lowered.