Limp home mode is a set of parameters the engine computer will use as a strategy to get you home without causing further damage to the drive-train of your car.
It means the computer noticed something wrong or lost contact with an important sensor that could ruin the engine or transmission and has de-rated the power so you can get home without the car stopping or damaging itself.
Limp Home Mode or Limp-in is an operating condition set by the vehicle computer. Technicians may refer to the vehicle computer as the ECU Engine Control Unit, ECM engine control module, or PCM power-train control module. The ECM or TCM transmission control module both have the capability of setting a LHM state. LHM limp home mode is a programmed response for the computer recognizing abnormal operation of your engine/drive-train. The intent is that it limits the power available and therefore will allow you to operate the engine and keep driving, but limit any damage to your engine and drive train.