L-carnitine is synthesized in the body from the amino acids lysine and methionine. It transfers long-chain fatty acids into the mitochondria where they may be oxidized to produce energy.
As great as that sounds, supplemental L-carnitine doesn't do a whole lot because your body can efficiently already create it's own. Supplementing with it certainly won't hurt but don't expect miracles. If you do decide to supplement with Carnitine, use the acetyl-l-carnitine version as it is the most bioavailable.