What does OILRIG stand for and what is the significance?

Oxidation Is Loss Reduction Is Gain = OILRIG

This is a helpful acronym to describe the term given to a loss or gain of electrons. If atom looses or gains an electron it becomes an ion, and the term to describe this process depends on whether an atom has lost (oxidation) or gained (reduction) an electron. 

For example, if Chlorine (Cl) becomes Cl- (a negatively charged ion we know it has gained an electron, since these carry negative charge. Therefore, we would say the chlorine atom has become reduced and a reduction reaction has occurred. 

Conversely, if Hydregen (H) becomes H+ (a positively charged ion, we know it has lost an electron, since it has become a positive ion. Therefore, we would say the hydrogen atom has become oxidised and a oxidation reaction has occurred.