Flasher Bulbs, or twinkle bulbs, are used to create the effect of animation to holiday displays.
If it's really quiet, you can often hear the slight ping they make when flashing.
Shown below are the traditional red, yellow, green, and blue colors.
Some Japanese brand bulbs of the 1970s included a pink color (not shown).

Twinkle Bulbs make use of a bi-metal strip for their flashing action.
A bi-metal strip is made of 2 metals bonded together.
When heated the strip bends, due to different expansion rates of the 2 metals.
The strip then moves away from a contact, and the bulb goes out.
When the strip cools and moves back to the contact, the bulb lights again and the cycle repeats.