Sealing a box means putting everything in a box and sealing it, which is a professional term used by the opera industry to express the end of the last performance of the year and to go out of business.
There are a lot of equipment for traditional Chinese opera performances, such as costumes, make-up props, stage props, etc., which often need to be packed in big boxes. When performing in a place, you need to bring these boxes. Therefore, when the last opera performance of the year is over, the actors need to put all the props and costumes into boxes and seal them up, indicating that they are closed for the New Year, and then open these boxes when they perform next year.