Burnt sailfish is the result of accidentally burning a raw sailfish while attempting to make a sailfish. Cooking a sailfish requires level 99 Cooking. They stop being burned at 99 Cooking with cooking gauntlets or the Cooking cape (or any of its variants).
The burnt sailfish has no use whatsoever, although you can unsuccessfully attempt to eat it. Like most other burnt food, it is tradeable to other players but not on the Grand Exchange.