The most common cause of a reduced engine power light to come on is a bad or dirty throttle body.
The throttle body could be bad or dirty and not allowing the engine to get the proper amount of fuel and air mixture to combust the fuel properly.
When that happens the engines power can be reduced causing your reduced engine power light to come on so you should get the engine checked out if the reduced engine power light comes on and doesn't go out after 30 miles or so.
Other causes of a reduced engine power light coming on are bad spark plugs, bad spark plug wires, bad ignition coils, low fuel pressure, bad fuel pump, clogged fuel injectors, bad fuel injectors or even a clogged or bad catalytic converter.
The reduced engine power light coming on is alerting you to something going wrong that needs fixed before it leads to other serious problems.