What does it mean when check engine light is flashing

It means there is a fault or multiple faults with the operation of the engine.
There can be many different causes/problems for the CE light to be triggered. It could be just one very minor issue such as failed oxygen sensor or or dirty mass air flow sensor in the air intake or something much more significant. You will not know until the appropriate scanner is hooked up to the car and the fault codes are identified. Quite possibly there will be multiple fault codes generated. The various fault codes indicate the type of fault the computer has identified but the cause or causes then need to be properly diagnosed before you can make any repairs or replace parts. Until the cause is identified and the fault fixed the CE light will continue to display.

You need to get the car to the dealership service center or to a competent mechanic with the correct scanning equipment for your car to identify the faults and the begin the diagnosis from there. Hopefully it will turn out to be something fairly minor and easily fixed.
All independent mechanics will have various scanning equipment but beware as they will not all necessarily be able to pick up all the fault codes being generated on your particular car and fix the fault or faults. (They may pick up one or 2, diagnose the fix for those , do the repair , think all is right and you still have the CE light displaying because there was another code being generated that their scanner did not read.). There can also be some fault codes generated for specific vehicles that are not generic to most cars but specific to a particular brand/model and the independents will not necessarily know what these codes mean unless they specialise in your vehicle model.

Don't delay having the vehicle checked and if in doubt take the car to the dealership service center for your car brand as they will have the latest scanning equipment to cover your model and will pick up all the fault codes being generated.