The command
tells Autolog to automatically hang up the modem if nothing transpires (no characters are sent or received) in talk mode for the specified number of seconds. If a bye file has been named, Autolog will execute the bye file before hanging up.
The idle option is handy if you forget you're accessing a remote computer and don't do anything for a long time in talk mode. idle will tell Autolog to automatically hang up the modem and return to command mode.