A crossover is when you take to things together and make a new story, for example:
say I take sky high and spider-man and make a story about a girl named April Parker.
She just got her powers and her parents want her to go to sky high to learn how to control them. As soon as she to the school she makes new friends. And some meets some bullies, but soon one of the bullies falls in love with her.
And that's basically what a crossover is.