The 555 is an eight-pin chip that turns a resistor and a capacitor into a time delay. It has been in production since the early 1970s and is still one of the most used chips in hobby electronics.
Two basic modes
Monostable: a trigger starts one output pulse of a fixed length. The length is about 1.1 x R x C, with R in ohms and C in farads. A 100 kilohm resistor and a 10 microfarad capacitor give roughly 1.1 seconds.
Astable: the chip retriggers itself and produces a continuous square wave. Two resistors and one capacitor set the frequency, which is how a 555 blinks an LED or sounds a tone.
Why beginners start here
It runs from a 9 volt battery, it is cheap enough that a mistake costs little, and a working circuit needs only a few extra parts.