A redox reaction is a combination of an oxidation reaction and a reduction reaction, this reaction involves the transfer of electrons from one species to another. When redox reactions are shown as half reactions you are able to determine what parts of the reaction are being reduced and what is being oxidized. Moreover, you are able to determine if the reaction is spontaneous or not once you see the half reactions. When an element loses an electron in a reaction it is considered to be oxidized however if it gains an electron or multiple electrons then it is reduced. In other words, if the oxidation number increases then it is losing electrons and therefore it is oxidized but if the number is decreasing then it is gaining electrons. Note that electrons have negative charges so this trend only makes sense because of this fact.
