Redox reaction is a reactions involving transfer of electrons between two systems such that both reduction and oxidation occurs. Reduction is the gain of electrons while oxidation is the loss of electrons.
In this case, if we write half balanced equations
Zn (s) = Zn 2+(aq) + 2e (oxidation)
2O 2- (aq) + 4e = O2 (g) (reduction)
therefore combining the two equations
2Zn (s) + 2O 2- (aq) = 2Zn 2+ (aq) + O2 (g) ( balanced equation)
