What is RPM?
RPM is abbreviated from Revolutions per minute, which means the number of turns in one minute. 

Normally, you can check the rotation speed by using a tachometer. (also called revolution-counter, tach, rev-counter, RPM gauge) If you need to measure the RPMs of your vehicle to help diagnose an issue, you may use a multimeter while you work under the hood.

Here's how to measure RPMs with a multimeter.

First of all, you need to have a multimeter, especially for the automotive use case. Take Trisco DA 830 for example, plug the Inductive pickup into the multimeter. Plug the black lead to COM jack and red lead to RPM INDUCTIVE jack. Turn the measurement-mode switch to the RPM position. 

After that, clamp the inductive pickup on the spark plug wire. Crank engine and now read the display. If the display is unstable, try to adjust the position of the inductive pickup.

You may have to divide the displayed reading by 2 if you use the multimeter on a 2 stroke engine vehicle.