This is equal to the amount of time (in server ticks, which default to 1/10s) since the player’s last action (such as executing a verb, moving, clicking an object, or selecting a topic link). This value is reset to 0 after each new action so you can use it to determine the time that has passed since the last one.
mob/verb/inactivity()
usr << "You have been inactive for [client.inactivity/10] seconds."