My first, too. I learn everything pokemon from my son. lol
Mega evolution is introduced in Gen 6. Basically, it's like a temporary, super-power-up form. It can have different types than the base mon. So, for Gyarados, Mega Gyarados will be a dual dark-water type, thus, making bite and hydropump both STAB.
You can read more about Mega here (many other sites cover it as well):