The term "Over 9000" was created when Nappa asked Vegeta what Goku's power level was, and Vegeta's scouter stated that Goku's power level was over said number. The term was made a meme, and was used many times as a joke.
In the original manga, the Japanese version of the anime, and even all its non-English translations, Vegeta actually says "It's over 8000!". However, even in the Remastered Uncut Funimation dub, Vegeta still says "It's over 9000!", now performed by Christopher Sabat. This change was made due to inconsistencies with the dubbing process, where 9000 fit the mouth flaps of the animation better.