L-carnitine is a compound produced in the body from the amino acids lysine and methionine, and plays a role in transporting long-chain fatty acids into the mitochondria where they're oxidised (burned) for energy. This biological function is the basis for carnitine's reputation as a fat burner. The evidence, however, is considerably more mixed than the marketing suggests.
The theoretical mechanism is sound: by potentially increasing fatty acid transport into mitochondria, carnitine could increase fat oxidation. However, oral carnitine supplementation faces a fundamental challenge: carnitine tissue uptake is slow and limited. Most oral carnitine is excreted in urine before it meaningfully impacts muscle carnitine levels. Multiple studies of carnitine supplementation in healthy individuals show no significant impact on fat loss or body composition versus placebo.
Where carnitine shows more promise: endurance performance research shows some benefit to carnitine supplementation in very long-duration events (>2 hours) where fat oxidation is a significant energy source. Carnitine L-tartrate (2g daily) may reduce muscle damage and soreness when taken before exercise. Carnitine combined with carbohydrates (which raise insulin and improve carnitine uptake) shows better results than carnitine alone in some studies. For most people pursuing fat loss, carnitine is not a worthwhile investment. The money is better spent on a higher protein diet, creatine, and caffeine - all with far stronger evidence for body composition improvements.