This interactive animation presents to you the functioning of a button.
You can click on the input button in order to display the effects on the logical state obtained on the micro-controller input.

• This is the 4th input of the PORTB (B3).

In this example, the switch is said to be connected in “pull-up” configuration: when we push the button, the input sees a logical 0.
This is called "inversed" logic