Your tie rods help keep your wheels and steering wheel connected so you can steer your vehicle properly.
When a tie rod does completely fail and breaks then you will lose your steering which means you'll not be able to steer the vehicle.
If a tie rod breaks while going down the highway at high speeds then you could roll your vehicle because you'll lose control of the vehicle.
If you're just driving slower then you'll just lose your steering and not be able to go anywhere until the tie rod gets replaced.
If a tie rod begins to go bad you'll usually get uneven tire wear on the front wheels and may also notice vibration.
Tie rods should really be replaced in pairs because when one tie rod goes bad the other tie rods is likely to go bad as well.