Mass and Energy are interchangeable, and vice-versa.

Can anyone tell me how I can make since of this? I have no idea what I am doingconfused

Newton's third law was "for every action, there is an equal and opposite reaction", which has very little to do with mass and energy being equivalent.

I don't think they were thinking about mass and energy being interchangeable back in Newton's day, the first notable person to begin quantifying it was Poincaré, who derived the equation m = \frac{E}{c^2}.

A few years later Albert Einstein (to who the equation is most often credited) published his fourth paper, containing the same equation in the form E=mc^2 and derivation of it. This describes the relationship between mass and energy.

It simply describes that there is an energy that you produce when you completely destroy a mass. You cannot destroy mass or energy, they both must be conserved, but in some cases they do seem to be destroyed. By making it a "conservation of mass-energy" law, we can see that mass and energy are not destroyed, but converted from one to the other.

One of the best examples is in the binding energies of a nucleus, the nucleus is lighter than the protons and neutrons that make it up, the missing mass has been converted to energy that is used to keep the nucleus together.

You must keep in mind that often the process is very inefficient, and therefore very little energy is often produced, instead the creation of other massive products is more favourable. The equation is more of a "theoretical maximum". In theory, the collision of a particle with its anti-particle should yield the full energy that corresponds with the masses.

I hope this helps you to understand mass-energy equivalence.

If you did want to talk about Newton's third law: "for every action, there is an equal and opposite reaction", then please reply saying so and I will go over that too :)