The electron volt (eV) is an amount of energy.
It is the amount of energy an electron would gain if it was accelerated through a potential difference of 1 volt.
1 eV = 1.6 x 10-19 joules (J) of energy
In atoms electrons orbit the nucleus.
There are particular allowed orbits where electrons can exist without emitting energy. Electrons can pass between these energy levels. When electrons are given enough energy to move to higher energy levels they are in an excited state, this is called excitation.
If an electron gets enough energy to remove the electron to infinity this is called ionisation.