Welcome to pin:out
The comprehensive GPIO pinout website for the BBC micro:bit
The comprehensive GPIO pinout website for the BBC micro:bit
P5 (the other being P11) is one of the two pins wired directly to the micro:bits onboard A and B push buttons.
This pin is pulled up to 3V via a 10K resistor (R22), and connected directly to 0V/GND when the button is pushed. Therefore it will read logic HIGH when not pressed, and logic LOW when pressed.