Strip on side of window letting cold air in
If you rent you can always ask the apartment manger the reason for the vent and if it can be closed.  Also look on the side of the window to see if there is any switches, levers or slides to open/close it.  If you don't own, you can always just cover it with 2 inch wide masking tape which will drastically cut down on the airflow and be easy enough to remove at the end of the lease.