A ceremony full of pomp and circumstance is one with lots of flashy, grand displays. Think presidential inauguration or the crowning of a queen.
Pomp itself usually means "a flashy, grand ceremony," but circumstance used to mean "fuss made about something," a sense we've mostly lost except in this phrase. In Othello, Shakespeare referred to "pride, pompe, and circumstance of glorious warre." The English musical composer Edward Elgar produced a series of marches called the Pomp and Circumstance Marches, and their popularity probably make this phrase an enduring part of the English language.