Why is Beijing Opera called Beijing Opera? What do these two words mean?

This can't be interpreted separately, and it's taken out of context. Drama is a kind of drama, and many kinds of dramas are called as such, such as Beijing Opera, Henan Opera, Pingju Opera, Yue Opera and so on.

Beijing represents the main dissemination and revitalization zone of such operas as Beijing Opera. During the Daoguang period, four Hui classes went to Beijing, which merged Hui opera and Han tune, and gradually formed Peking Opera, which was widely popular in Beijing.

For example, Henan Opera, short for Henan, is the most popular in Henan.