Many cars have a cam belt or timing chain fitted which is a part of the internal combustion engine that synchronizes the rotation of the crankshaft and the camshaft(s). This allows the engine’s valves to open and close at the proper times during each cylinder’s intake and exhaust strokes.
This belt will often need replacing over time and the interval for this is different for each make and manufacture of vehicle. If you do not change the belt and it breaks or the tensions fail that support it then you could ruin your engine and often the damage is more than the cost of the vehicle, especially for older vehicles. The cost of replacing the cam belt again varies from vehicle to vehicle costing anywhere from £150 upwards of £400, but can mean the difference between having a working car or a heap of scrap if it goes wrong. If buying a car be dubious to believe that the cam belt has been changed unless you can see a receipt for it.