A tether is a a strap that is attached to the top of your car seat. It has a hook on one end of the strap.
After you install your car seat with a seat belt or LATCH, connect the hook to a tether anchor in your vehicle.
A top tether works by keeping your child's car seat back so his or her head won't move as far forward in a crash. This can reduce the risk of head and neck injuries. This photo, from the University of Michigan Transportation Research Institute (UMTRI), shows a crash test of a car seat with a top tether and one without. In the seat without a top tether, the crash test dummy's head moved forward six inches more than the dummy in the car seat with a tether.
As of September 1999, all new child safety seats with forward-facing harness straps have been manufactured with top tethers.
Vehicles manufactured after September 2000 have top tether anchors in three seating positions. Read your vehicle owner's manual to learn where to find the top tether anchors in your car. Some tether locations are marked with a symbol like this.
If you have an older vehicle, check with your vehicle manufacturer about having tether anchors installed.