The reduced engine power indicator is letting you know that something is wrong with either the engine or the other electrical components of the engine.
The engine is then put into the reduced engine power state which slows the engine down and reduces your engine and driving speed so that you can limp the engine and vehicle home or to a mechanic to have it checked out.
Some things that might cause an engine to go into reduced engine power mode are a bad battery, bad alternator, transmission problems, low engine oil level, low coolant level, timing issues, bad ignition coils, bad spark plugs etc.
Usually it's due to something wrong with either the engine or transmission and you need to get the engine checked out by a mechanic or your dealer right away before driving too far with the reduced engine power.
Whatever it is it needs to be fixed before more damage is done to the vehicle.