The c² is a constant: the speed of light squared. The formula says that energy and mass are equivalent. They’re the same thing.
People will quibble and nitpick, but these matters were settled more than a century ago. Matter can change into energy, energy can create matter. Energy can travel at the speed of light, matter cannot. It’s not worth the strain to try to understand how they could be the same thing if they’re so different.
There’s math involved, and ideas that don’t fit easily within a human mind, that’s all. Let Ethan Siegel do your thinking for you. He gets it. Anyway, energy and matter are two ways of looking at the same thing. You just have to bend your mind a bit.
Here’s a video, a silly one, that discusses the very beginning of our universe:
Enjoy the universe, and godspeed.