The 555 timer gets its name from the three 5 kΩ resistors it uses to generate the two comparators reference voltage. It is a very cheap, popular and useful precision timing device that can act as either a simple timer to generate single pulses or long time delays, or as a relaxation oscillator producing stabilized waveforms of varying duty cycles from 50 to 100%. In this tutorial, the presenter will demonstrate how to setup 555 timer circuit in monostable mode. This will allow an LED to be turned on for a specific duration after pressing a button. The time the LED stays on can be changed by changing the resistance and capacitance in the circuit.
Courtesy: Circuit Basics
The 555 timer chip is extremely robust and stable 8-pin device. It can be operated either as a very accurate Monostable, Bistable or Astable multivibrator to produce a variety of applications. Some of the applications include one-shot or delay timers, pulse generation, LED and lamp flashers, alarms and tone generation, logic clocks, frequency division, power supplies and converters etc. In fact any circuit that requires some form of time control uses the timer IC as the list is endless.
More tutorials available here.