RPM, or revs per minute, measures the speed of your engine, rather than the velocity of your vehicle. It is displayed on a tachometer located on the instrument panel.
Most drivers should be aware of the movement of this dial, and that as we push down with our right foot, the engine sings (or groans) and the rev counter rises in tandem.
What it is measuring is the number of thousand times the crankshaft is turning per minute, thus pushing the engine and driving the numerous shafts linked to it, until the wheels turn.
The redline on the tachometer shows the maximum rpm: in cars with a rev limiter you can't accelerate past the redline but have to upchange gear.
RPM measures the true work of the engine itself, regardless of what speed you are doing. In petrol engines the range of revs varies between 5,000 to 7-8,000 before the engine cuts out, or explodes. In diesels this range is lower, around the 4-5,000 mark in most regular saloons.