A 555 timer is an integrated circuit used for generating precise timing and oscillation. It can operate in different modes, including monostable, astable, and bistable, allowing it to function as a timer, pulse generator, or flip-flop. The versatility of the 555 timer makes it popular in various electronic applications, from simple timers to complex circuits. In monostable mode, the 555 timer produces a single output pulse when triggered, while in astable mode, it continuously oscillates between high and low states, creating a square wave. Its ease of use and low cost have made the 555 timer a staple in electronics projects and education.