AC power, by definition, is where the current flows back and forth in the circuit, and as such, it will switch off momentarily as the current passes through zero. Above a comparatively low frequency, the human eye cannot see the interruption, and interprets what it sees as a steady light. A 555 can drive a LED with a variable nark-space ratio to give a dimmable Led.
The whole effect is very similar to a cinema film - what is actually projected is a series of still pictures (24 or so per second is common) but the projected film is seen as continuous motion.
Pulsed LEDs on DC supplies may be the designer pulsing the LED at a given frequency and relying on the eye's 'Persistence of Vision' (as it is called) not to notice.
Hope this helps