In ancient China, the emperor would marry many concubines. However, only one of them can be crowned as Empress. In other word, the Empress was the officially recognized wife of the emperor. Generally, the Emperor would be responsible for the affairs of the nation while the matters within the Hougong (Back Palace) which related with the concubines of the emperor would be responsible by the Empress.
Basically, after the death of the Emperor, his Empress would be crowned as Empress Dowager in return. Anyway, the Prince who ascended the throne of the Emperor might not necessary be the son of the Empress. However, if the siblings of the Emperor ascended the throne, then the Empress would not be crowned as Empress Dowager but just crowned with the Emperor’s name in front.