Rotations Per Minute, often shortened to just RPM, is a measurement used to determing engine speed. Since engines all have certain maximum speed they can operate at, that point is considered the "redline", because of the red mark on the RPM gauge.
Revolutions or Rotations?
Sometimes RPM is called "Rotations Per Minute", which isn't entirely accurate, since I rotation isn't always 360°, while a revolution is always that amount.