A Tintinnabulum (roughly "little bell" in Medieval Latin) is a bell mounted on a pole, placed in a Roman Catholic Basilica to signify the church's link with the Pope.
The tintinnabulum consists of a small gold bell within a golden frame crowned with the papal tiara and Keys of Heaven. If the Pope were to say Mass within the Basilica, the tintinnabulum would be used to lead the very special procession down the shrine's aisle. However, these symbols are not stipulated in the 1989 Vatican directives.
In the Middle Ages it served the practical function of alerting the people of Rome to the approach of the Pope during papal processions.
Interestingly, the word is attested in the Appendix Probi as "tintinabulum" castigated by the author as an incorrect Vulgar Latin form of the Classical "tintinaculum".