A flashing check-engine light means an active engine misfire severe enough that raw, unburned fuel is passing into the exhaust and igniting inside the hot catalytic converter, which can melt its internal substrate within minutes of continued hard driving. Ease off the throttle, avoid hard acceleration or towing, and get the car scanned the same day if possible - do not just clear the code and keep driving, since that turns the light off without fixing the misfire.
Roughly in order of likelihood: worn or fouled spark plugs (most common, $100-$450, P0300-P0304); failing ignition coils (common, $100-$450); a vacuum leak leaning out one or more cylinders (occasional, $100-$800); a clogged or leaking fuel injector (occasional, $100-$450); low compression from mechanical wear (less common, $100-$800); and, as a consequence of driving on an unaddressed misfire, a damaged catalytic converter (P0420/P0430, $150-$2,400). P0301-P0304 point to one specific cylinder, making that cylinder's coil, plug, or injector the prime suspect, while P0300 (random/multiple) more often points to something affecting several cylinders at once.