I’m curious about some aviation signals.  What does it mean when a plane is flashing (alternating) its landing(?) lights?  I’ve also seen helicopters flash a single (centrally located) light.  

Also, is there a good website that will answer these kinds of questions for non-aviators?

It might not mean a thing, and probably doesn't.  Some A/C, (as well as motorcycles) have an automated flasher built into the landing lights for collision avoidance purposes.  At night, the three position switch is placed in the "constant" on position for landing.  Daytime it is set to "Flash".

Cavu

ETA:  You might try this site.  I don't know how much help it would be, but you are sure to get more people with "aviation" expertise there than here.Landings.com

On the high seas, flashing landing lights mean Attention and/or Follow Me when directed at a surface vessel.
In  my Cessna I used to keep the landing and taxi lights on for traffic avoidance in crowded airspace.

Fritz

Flashing lights on aircraft are for one thing, visibility...to be seen.

flap

In the event of a communications failure, flashing the landing light is a way to aknowledge light gun signals from the tower at night. It the same as rocking the wings to aknowledge during the day.

A flashing landing light can be used to acknowledge a light gun signal, it can be used to signal a communications problem (but they would also put a certain 'squawk' in the transponder).  On some planes, they automatically flash on/off for collision avoidance.

Other lights you'll see on aircraft are position/navigation lights (steady red on left wingtip, steady green on right wingtip, and steady white on the top of the vertical stabilzer trailing edge).

Anti-collision strobe lights generate white flashes on each wingtip and at the most aft point of the fuselage.  An anti-collision rotating beacon is located on the top of the vertical stabilizer and has a red flashing light.

And many aircraft have separate landing lights and taxi lights (both of which are white lights).  Many landings lights can not be used on the ground for too long due to overheating, hence taxi lights.

Edit: typo...

Lots of reading material for you here....
www.faa.gov/ATpubs/AIM/

and here:
www.risingup.com/fars/info/91-index.shtml

We have ALLS [Alternating Landing Light System] on our jet. As flaperon pointed out it's all about visibility. Being seen by other aircraft, etc. And theyre on a squat switch, no flash on the ground..
Oh yeah we arent supposed to use them for landing... lol

myitinaw

Disco lights. Lets ya see em easier.