This is a bottle of Racers Edge Carb Sealant for Nitro engines. If your nitro engine has an air leak around the neck of the carburetor, or the needle valve assembly, it can make the engine run erratically and change from lean to rich conditions. To avoid this problem it is always a good idea to seal up the carburetor around the neck and needle valve assembly, and this sealant is excellent for that purpose. You can also use it to seal the back plate of your engine to avoid air leaks around the back of the engine as well.
This is similar to the Racers Choice Air Seal carburetor sealant we used to sell under the part number RCC7039.