Tie rods are car parts that are a crucial part of your steering mechanism. Known as track rods in the UK, they connect the steering center link or rack gear to the steering knuckle. This then means they run under the car and shorten and lengthen in coordination with the turns of the steering wheel in order to ensure that the wheels face in the same direction as the driver intends to drive.
In fact tie rods are not specifically car parts but instead the term is used to refer to any narrow rods with tensile strength. The radius of the tie rods when compared to their length means that they are strong in terms of tensile strength but would buckle under any other kind of pressure. Other uses for tie rods are in the spokes of tires or in suspension bridges, though these are of course different in a number of ways to those used in cars.
If a tie rod needs replacing then the steering of the vehicle may wander or become erratic and tires would wear more quickly. It is thus necessary for them to be checked yearly and replaced when required.