In biology, an organ (from the Latin "organum" meaning an instrument or tool) is a collection of tissues that structurally form a functional unit specialized to perform a particular function. Your heart, kidneys, and lungs are examples of organs.
Organ is the name we give to functional elements in the body and quite often they are also physically distinct such as your kidneys, your liver, your heart. Those are all defined by a shape and a location in your body and they are also defined by a function. Your stomach and your gastrointestinal tract are both organs and they process food. Your lungs are also organs and they exchange oxygen and CO2. And your heart is another organ which is essentially a pump.