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A "glossary" is a collection of "glosses," or explanations of a word or term. The Leiden Glossary contains glosses and commentaries by two priests and scholars, Adrian of Canterbury and Theodore of Tarsus (mentioned here), who were both at St. Augustine's Abbey in Canterbury.
The 48 chapters are lists of sayings and phrases used by Adrian and Theodore in their teaching, as well as commentaries they made on other works: think of it as a teacher's handout to his students so they don't have to take notes. There are, for instance, 8 chapters by Theodore with glosses on the "Pastoral Care" of Pope Gregory I (Gregory has been mentioned here).
There are also glosses from different people. For instance, there are three glosses on the same subject of the Historia Ecclesiastica ["History of the Church"] of Eusebius. The three are of differing quality, as if the book records the attempts by three different scholars—maybe students— to explain the passages in Eusebius. One of them echoes a different commentary found elsewhere that is known to be by Aldhelm, so it may have been Aldhelm himself who contributed it to the Leiden.
The Leiden is a mixture of glosses in Latin and Anglo-Saxon, another indication that the original glossary must have come from England. The Leiden Glossary was made in the library at the Abbey of St. Gall, presumably from that original. One of the things that we learn collaterally from the Leiden Glossary—because of the manuscripts about which the glosses have been written—is that the library at St. Augustine's Abbey must have been extensive. Alas, it did not survive the dissolution of the monasteries under Henry VIII.