A 555 timer is an example of an integrated circuit, or an "IC chip." It is an electronic component consisting of a small piece of silicon or other semiconductor material with an electronic circuit built on it.
The 555 timer gets its name from a set of three resistors in its circuitry, each having a value of 5,000 ohms. The chip has three modes or ways it can be used: monostable, astable and bistable. Each mode generates pulses used by engineers to build circuits such as game paddles, tachometers and waveform generators. The 555 timer is also used by hobbyists in designing fun projects such as metronomes and toy organs.