L-Carnitin is an amino acid which is produced by the liver. The body can produce only a small part of the L-Carnitin so the larger part has to be taken in through food sources. Meat, for example has a high content of L-Carnitin.
L-Carnitin acts as a transporter within the body by transporting fat from the cells directly into the mitochondria where it then gets burned.
Athletes and physically active people can't always cover their demand of L-Carnitin just through normal food intake so for them, supplementation is a good option.
During physical activity, the body produces the required energy first from stored carbohydrates. Only once these depots are depleted, will fat be used as a source of energy and this is when L-Carnitin is most efficient.