Oxidation is a reaction where an atom, ion or compound loses an electron. It always happens at the same time as a reduction reaction - where an atom, ion or compound gains an electron. Usually, the element reacts with oxygen in an oxidation reaction, although not always.
Example: Hydrogen + Flourine --> hydrogen flouride. Here the hydrogen is giving electrons to the flourine. The hydrogen is oxidised, the flourine is reduced.
Oxidation Is Loss, Reduction Is Gain (of electrons)