In computing, an Easter Egg is a hidden trick that an application or web site can do. There are a lot of famous easter eggs buried in various programs, and Google is no exception.
If you have a reasonably modern web browser, search Google for do a barrel roll. The Google screen itself should rotate through 360 degrees in imitation of a barrel roll. Here are some example screenshots of the rotation:
do a barrel roll
If you want to see the barrel roll recorded on video, try this YouTube video:
The do a barrel roll phrase was popularized by 4chan, but originated from Star Fox 64, where a major character repeats that phrase to encourage dodging from enemy fire. See Know Your Meme for an explanation or this YouTube video for gameplay footage: