Annihilation

Whenever a particle and its antiparticle meet they annihilate each other. Annihilation is the process by which mass is converted into energy, particle and antiparticle are transformed into two photons of energy.

Mass and energy are interchangeable and can be converted from one to the other. Einstein linked energy and mass with the equation:

You can think of it like money; whether you have dollars or pounds you would still have the same amount of money. So whether you have mass or energy you still have the same amount.

The law of conservation of energy can now be referred to as the conservation of mass-energy.

The total mass-energy before is equal to the total mass-energy after.