A tachometer shows how fast an engine is turning over, in revolutions per minute. How that relates to miles per hour on the speedometer depends on which gear is translating the engine's revolutions to those of the wheels. RPM information is most useful with a manual transmission.
The car is heading toward a higher gear -- for instance, accelerating to cruising speed. When the gear shifts, RPM will fall again.
A drop in RPM can indicate that the car needs to shift down -- for instance, to climb a steep hill. After rapid acceleration, it can also indicate that the car has settled into its cruising ratio, where the engine doesn't have to turn so furiously to maintain steady vehicle speed.
If you notice your tachometer shows a different reading in a particular situation, or wobbles up and down without reason, it could be a symptom of a problem you should explore with your mechanic. Don't assume it's something wrong with the gauge itself; speak knowledgeably about your concern for a problem in the engine's communication with the wheels, and be prepared for a transmission or timing-belt repair.
