That's the vent line for the battery.
Wet cell batteries have a nipple connector on the Positive(+) terminal side where this vent line connects to so that chemical vapors don't build up inside under the seat.
It's also important to unlatch your seat and move it to the side or set it towards the rear to let any accumulated water inside the bilge bay to properly vent out and to guard against rusting if the seat is left latched after rides.