Very simply put, embalming is preservation of a human remains by replacing blood with an embalming fluid. Embalming may also serve as a way to sanitize the remains.
In some instances, embalming is not required by law. However, if you want specific sorts of services for example a service with a public viewing, you may have to select embalming. Your state may also have timelines for when embalming has to occur, especially if no refrigeration is available.