When listening for heart sounds you use the bell of the stethoscope.
The bell of the stethoscope is used for listening to low frequency sounds while the diaphragm of the stethoscope is used for listening to higher frequency sounds.
The bell is best for detecting lower pitch sounds, like some heart murmurs, and some bowel sounds.
It is used for the detection of bruits, and for heart sounds (for a cardiac exam, you should listen with the diaphragm, and repeat with the bell).
The reason a stethoscope has two different sides for listening is one side of the stethoscope is used to listen to low frequency sounds and the other side of the stethoscope is used to listen to high frequency sounds.
The bell end of the stethoscope is the end of the stethoscope that is used to listen to low frequency sounds.
The other side of the stethoscope which is the diaphragm is used to listen higher frequency sounds.
Most times doctors use the larger part of the stethoscope to listen to your stomach and heart and the smaller side of the stethoscope to listen to your wrist and pulse when taking your blood pressure.
So one side of the stethoscope lets you listen to higher frequency sounds such as the heart and the other side lets you listen to lower frequency sounds such as your pulse.