Electrical impulses are generated in the right atrium's sinus node, and the output is normally a regulated, even pace to contract ventricles and is represented by heart rate.
An abnormal EKG would indicate the pace of the impulse to be too fast, slow, and sometimes the pathway for the impulse to reach the ventricles for contraction is blocked causing a skipped beat or the impulse takes an alternate pathway causing a rhythm disorder (irregular heartbeat), etc..
Impulses are measured and calibrated with the EKG instrument. As an example the electrical impulse encounters a thicker than normal heart wall and as a consequence it takes longer for the impulse to pass and that is represented by a longer than normal wave interval, or the impulse may meet higher than normal resistance causing a higher amplitude for the impulse. The location is determined by the angle of the electrodes placed on the individuals chest., etc., etc.