What is the electrocardiogragh monitor showing the nurses and doctors?

The ECG paper records time sequences and amplitude of the electrical activity of the heart.
The horizontal lines measure time intervals and heart rate.You will see first the P wave, the QRS complex, and the T wave. The P wave is the atria contracting, the QRS is the ventricle contacting, and the T wave is the relaxation of the ventricles.
They can see just how each part of the heart is working, how fast it is beating and where there may be any problems.