However, the movie was to be named Krrish, and hence, they showed in the first part of the movie how the Superhero played by Hrithik Roshan got the name.
As per the movie, Hrithik Roshan's character is called Krishna, which is a popular name in India, due to Lord Krishna. As such, krishna is Sanskrit for dark or black, and hence, the dark skinned God was called Krishna.
Anyway, so this character was called Krishna, and he has some superpowers which he has promised to hide from the world. But one day when he sees a kid stuck in fire, he saves him, but the kid wants to know his name first, and he starts telling him. He had only said Krish.. when he realized he couldn't give away his identity, and so, he stops there. And hence, he's called Krish, which was later spelt as Krrish.