A Rocket Start is a speed technique introduced in Super Mario Kart and featured in every Mario Kart game since. It allows players to burst out of the starting line with blazing high speed, gaining a head start over any racer that simply drives out of the starting line. Similar to Mini-Turbos, Rocket Starts all vary slightly from game to game. You must press the Acceleration button at different times during the Pre-race countdown. Super Mario Kart's version had to be prepped right after the 1st light, and Mario Kart 64 and Mario Kart: Super Circuit was the exact opposite, on the latest light. From Mario Kart DS and onward, Rocket Starts can be performed by holding A right after the 2 on the countdown fades.
The technique was given a name in the instruction manual for Mario Kart 64.
Performing the Rocket Start to early then when it should be started will result in the kart stalling, so proper timing is needed, or you can get a really bad result.