The 555 is a single-chip version of a commonly used circuit called a multivibrator, which is useful in a wide variety of electronic circuits. The 555 timer chipis probably the most popular integrated circuit ever made.
You can use the 555 chips for basic timing functions, such as turning a light on for a certain length of time, or you can use it to create a warning light that flashes on and off. You can use it to produce musical notes of a particular frequency, or you can use it to control positioning of a servo device.
Here is the arrangement of the eight pins in a standard 555 IC. The 555 comes in an 8-pin DIP package.