What is a Bull?
"Bull" refers to the metal seal that authenticates the document, much as we authenticate documents with signatures. The practice dates back to ancient Sumeria, where impressions were made in clay in order to track and authenticate transactions. This practice remained unchanged for 4,000 years. It was adopted by the Romans, who used lead rather than clay. The lead was heated to soften it to create the impression, hence the word "bulla" which derives from the Latin word meaning "to boil."
Papal Bulls took on a standardized form, with representations of Saints Peter and Paul pressed into one side of the Bull, and the name of the Pope issuing the decree on the reverse. Below see a close-up of the Bull authenticating this document.
What is this document about?
The matter at hand concerns the alienation of Church property. There was a monastery that had a parcel of land it wished to sell; however, selling Church property required Papal permission. Holding on to ecclesiastical property was a matter of great importance to the medieval Church, since it was the economic foundation of the Church's activity. The general principle is that a property, once given to the Church is in effect given to God, and no one can legitimately give it to anyone else. The issue here appears to be a relatively minor matter of trading a small and distant field inherited by one of the monks; hence this document delegates the Vicar General (who acted as a kind of business manager) of the diocese of Lodi, the local church authority, to investigate the matter and give the papal permission if he deems it right.