When someone is over the moon, they're really delighted or happy about something. If you think your friends forgot your birthday, you'll be over the moon when they all jump out and yell, "Surprise!"
The idiom over the moon has been around for at least a hundred years, though it became especially popular in the mid-1900s. It probably comes from a line in the 18th-century nursery rhyme Hey Diddle Diddle: "The cow jumped over the moon." Being over the moon about your new bike means you're absolutely thrilled about it, and full of the kind of energetic enthusiasm that might allow a fictional cow to actually jump over the moon.