Debouncing button or switch inputs on microcontrollers ... some other capabilities to inputs you might want to look at this interrupt service routine (ISR) method from [Lee] aka [stockvu].

Arduinos can have more interrupt pins enabled by using pin change interrupts. In ATmega168/328 based Arduino boards any pins or all the 20 signal pins can be used as interrupt pins. They can also be ...

The concept of Interrupts come into picture in the scenario ... During this time, the Arduino processor cannot do any other tasks like read from a button, for example. Here comes millis to the rescue.

I made this Arduino 8 bit binary led counter as a solution for ... debounce the button and increase the value of presses each time we press the button (when we trigger the interrupt from the setup()).