Heart rate or pulse is one of important diagnostic and monitoring parameters in human body. If you are doing exercises you may also need to monitor heart rate in order to determine its effectiveness. Ohoilett shares pretty simple circuit that may be used to detect pulse. The idea here lies on simple method which is based on measuring passing light through the finger. In biomedical engineering this is called photoplethysmography. During one heart period blood is pumped through the body, so capillary volume increases and decreases periodically. By shining light through the finger we can detect its variation and so heart rate.
In order to have reliable data wee need to condition incoming signal to Arduino input level. So there are couple operational amplifiers used where first one takes current signal from photodiode and converts it to voltage. This stage is called trans-impedance amplifier. Then signal is passed through high pass filter which is made of RC circuit. And last stage is gain stage, which amplifies signal to Arduino input level.
All is left is to detect signal peaks within software and represent it in one or another way.