Shoutbox

If you're talking about the feature in Messenger (Tools > Options), then it considers you idle when you don't use your mouse and keyboard for the specified amount of time.

If you're talking about the feature in Plus (Plus! > Preferences > General Options > Miscellaneous > Automatically change status after a period of inactivity) it's the same, the only difference is that it lets you choose which status you want to change to.