There’s nothing ordinary about it.
The Glossa Ordinaria is, simply put, an extensive medieval collection of biblical commentaries. Literally speaking, a glossa is an annotation, originally found in the margins of the biblical text, that some copyist would add to clarify the meaning of a word in either Hebrew or Greek —the word glossa being originally Greek itself, meaning “tongue” and, by extension, “language.” Most of these marginal commentaries helped standardize the biblical text and the ways in which biblical languages themselves were transmitted.
The collection is referred to as ordinaria (literally, “ordinary”) because it soon became the standard commented edition of the bible from the 13th century on. True, at first the annotations were just used to clarify the meaning of a word, but in the course of time they began to include scriptural, hermeneutical commentaries, mainly from the Church Fathers. In fact, early Christian authors like St. Jerome generously included and used glosses in their own translations and commentaries of the sacred text. As seen in the featured image, the biblical text would be in the center of the page, surrounded by commentaries from different authors explaining and discussing its contents, like in the Talmud.
Now, the idea of an extensively commented edition of the biblical text is not a Christian invention. Commented editions of any text were quite popular during the Middle Ages. The classic Mikraot Gedolot, also known as the Rabbinic Bible, is an edition of the Hebrew bible (in Hebrew) that includes commentaries from prominent Jewish bible scholars, and was compiled around the same time of the Christian Glossa Ordinaria.
The name, Glossa Ordinaria, was broadly used in the Middle Ages for any standard commentary on any important text —most of them juridical. But since the most commented and more widely used text then was the bible, the term Glossa Ordinaria was understood to refer to the standard commented edition of the Bible, whenever used without any further specification
Originally, the compilation and arrangement of the Glossa was wrongly believed to be the work of the 9th-century German Benedictine monk Walafrid Strabo. Later research showed it was the 12th century French theologian and Christian biblical hermeneutics pioneer Anselm of Laon who gave the Glossa its initial impetus, until it was printed for the first time in 1481 in Strasbourg, with the title Biblia Latina Cum Glossa Orden Walafridi Strabonis Aliorumque et Interlineari Anselmi Laudunensis.