A popular internet meme from the anime DragonBall Z.
This concept originated in Dragon Ball Z when Goku was surprisingly, beating up Nappa. When Nappa asked Vegeta what Goku's power level, Vegeta responded that it was over nine thousand.
This concept is mainly about the protagonist surprising their enemies with their new power level. It usually occurs after a training arc and the protagonist is now able to defeat an enemy (sometimes without a hassle) who they would not stand a chance against before.
This also tends to be in a lot of Dragon Ball spoofs or anime comedy because it is quite popular.