Over 9000 is a term used to express a great amount of skill, strength, speed, etc. The term originated from the anime show Dragonball Z when Nappa asks Vegeta what his scouter ( a device used in Dragonball to measure the power level, or battle strength, of someone) said about Goku's power level. Vegeta's scouter was designed to pick up a power level up to 9000 whereas Goku's power level was "over 9000" which was what he told Nappa.
*Goku shows up*
Nappa: Vegeta, what does the scouter say about his power level?
Vegeta: It's over 9000!!!!!
*Vegeta removes and crushes scouter*
Nappa: What, 9000?! There's no way that can be right!