When substances react with oxygen, they form new substances called oxides. These reactions in which a substance gains oxygen is called oxidations.
Sometimes, it is possible to remove oxygen from an oxide. For example, if mercury oxide is heated strongly, it will decompose to mercury and oxygen.
Mercury oxide -> mercury + oxygen
Reactions like this in which a substance loses oxygen are called reductions.
Reduction is the loss of oxygen. Oxidation is the gain of oxygen. Also, reduction is the gain of electrons. Oxidation is the loss of electrons.