Most hear rate monitors have two parts: a strap-like piece that you wear around your chest and a receiver unit that displays or records data.
The monitor piece worn around your chest has sensor pads that must be in direct contact with your skin for the unit to work. Data from the monitor piece is usually sent wirelessly to the data display / recording piece.
Data Display / Recording Piece
The data display piece receives heart rate signals from the monitor piece.
Forerunner 301 has a built-in receiver that works with the monitor strap provided with the unit. It must be within about 3 feet to receive signals from the monitor strap.