Um...well. My people are a warrior race called the saiyans. Super saiyan is a transformation that temporarily makes a saiyan about fifty times more powerful than they normally are.

[Then there's all the other kinds of transformations but she's got no idea how to explain the differences between all of them, beyond "some of them are more powerful than others".]

Hair and eye colour change with the transformation and there's glowing energy around the saiyan, at least at first.

[She hopes that's a good enough explanation. If not, she can always show him. She's practiced a few times now and probably won't turn into her scarier form by accident.]