Sixteen pins you can give a meaning to. Any of them can be an output you drive yourself (relay, LED, fan) or an input you read (push-button, switch, PIR, reed contact) — the whole slot is set here: which GPIO it is on, what it is, and what it does. An input runs any action a button can, so a wall switch reaches the whole radio; a slot that starts a rule is bound on the rules page.