A sealer is a liquid coat that is usually a clear/transparent finish that coats the outside surface of the wood. A sealer’s main purpose is to keep moisture out. if makes wood water-resistant which is quite helpful, as moisture may cause fungal development and wood decay.
A primer is the first complete coat of paint applied to the wood. Primer creates a layer that paint can better adhere to. A primer is required if you want substantial color changes or if you are using bare wood.