Burnt crab meat is the result of accidentally burningcrab meat while attempting to make cooked crab meat. Cooking a cooked crab meat requires 21 Cooking, and burning it will become less common with a higher Cooking level.
The burnt crab meat has no use whatsoever. Unlike most burnt food, it is not tradeable. It is also one of the only burnt food items (along with burnt jubbly and burnt fishcake) to not stack. You can attempt to grind it, presumably to make ground crab meat, but this has no effect.