Limp mode is an engine mode that is designed to place absolute minimum stress on a engine while still providing sufficient operation to provide limited locomotion. An ECU normally puts an engine into limp mode when it knows / has senses an engine fault that could be made worse by the driver continuing to drive in their normal manner. In effect reducing the amount of harm that an engine will suffer. So you can think of limp mode as a self defense system for engines that is deployed by the ECU to reduce damage until the vehicle can be repaired.
The sensors on a vehicles DPF can supply such signals that cause the ECU to issue a ‘Limp Mode’ state to the engine. This can happen when the ECU senses that the DPF charge (amount of particle matter collected) is high and for whatever reason the ECU can not either set the engine to a DPF regeneration mode or it has and the DPF charge has not reduced by way of particle burn off.
Many vehicles display a ‘Regen’ light on the dashboard when a DPF regeneration is required. Under normal operation the vehicles ECU should take care of this and the ECU will set the engine into a DPF regeneration cycle. Some manufacturers recommend that the vehicle be taken on a long run / experience motorway driving conditions. This has the same effect as the ECU setting an engine into a DPF regeneration mode, in both cases higher than normal exhaust tempretures are experienced by the DPF, this should result in DPF cleaning via particle burn off.