Why is a large flashing "X" placed on a closed runway at Reagan National Airport?
The standard way of marking a runway as "CLOSED" is to put a big giant X on it - for fields which operate at night that X needs to be lighted. The specifications for the runway closure marker are outlined in an FAA advisory circular (AC 150/5345-55A)

The purpose of the marker is to indicate that the runway is temporarily closed (either administratively, as is the case at DCA, or for construction/repair/maintenance).  It is redundant with air traffic control measures (such as NOTAMs) indicating that the runway is out of service, in case some pilots didn't check NOTAMs or somehow managed to stumble into the airspace and happen upon the airport while it's closed.  (Obviously in the case of DCA they would have other problems were they to wander blindly into the area without talking to anyone, but believe it or not it happens far more often than any of us would like.)



The big flashing X doesn't "stop" you from landing on the runway - it just tells you you really, really shouldn't be landing there. There is a United States Senator who has achieved some notoriety in the aviation community after landing on a closed runway.

The X is also quite a bit bigger than you think it is to ensure it's visible from the air  (the arms are 14 feet long):


(I was looking for a good aerial photo of one at night, but unfortunately couldn't locate one. The intent is for the X to be about the same size as runway numbers so there's no good reason for a pilot to not see it.)