Limp Mode is a default set of values stored in the ECU non volatile memory. If the ECU senses a sensor value missing or out of programmed range, the CEL will illuminate and the ECU will shift to these values to allow the engine to run at reduced power to "limp" to a service facility without further damage to the engine.
Depending on the engineered values and type of engine, the limp mode can enrichen the mixture, change the timing curve, limit boost, etc. The ECU should remain in limp mode with values locked until all signals are again received within the programmed ranges and the CEL will extinguish.