What do you mean by oxidation and reduction?

Oxidation: it is the removal of electrons from an atom or ion.

For ex:

Na(s) → 〖Na^+〗^ (g) + e^-

 

Reduction: it is the addition of electrons from an atom or ion.

For ex: 

〖Cl〗_2 (g)  + 2e^-   →2〖Cl〗^-

In oxidation the oxidation state of the atom increases whereas in reduction the oxidation state of the element decreases.