What is mc2? And how is it related to the speed of light?

M represents units of mass, and c2 is the speed of light squared, or multiplied by itself. it is equal to energy.

the equation says that energy and mass (matter) are interchangeable; they are different forms of the same thing.

The reason how it is related to the speed of light is that energy, be it light waves or radiation, travels at the speed of light.

This is usually used in the equation e=mc²

In this equation e is energy, m is mass, and c is the speed of light. So, to answer your question, mc² is mass times speed of light squared and it is related to the speed of light due to c