Tappets are the part of the engine that pushes the valves open. When not set correctly they sound like a ticking 'purring' noise that changes with engine speed. If you don't get this repaired reasonably soon you will cause more problems and probably more damage to the engine. You will also get Poor fuel consumption and poor performance.

Lifters and tappets are the same thing. A mechanic can tell from the sound and the location. They are adjustable in some cases and are prone to wear that causes the ticking sound.