A set amount of RPMs which your engine can put out before you pretty much blow it up. This is usually where your numbers on your tachometer start turning red, just be sure to keep it out of the red or else $$$$ will be needed. For instance, the redline on my 95 Jeep Grandcherokee is 5,300 RPMS that is exactly where they start turning red, some cars have a rev limiter on them, so due to my rev limiter the engine will not go past 5,300rpms and the engine starts "bouncing" meaning the needle will do the same. I advise you not to try this.
Dude your not supposed to shift while ur racing till you redline it