1. Literally, bounteous mother, a title given by the Romans to several goddesses associated with abundance.
2. "Nourishing mother" or "foster mother".
3. The school, college, or university that someone formerly attended.
4. A song used as the anthem of a school, college, or university.

The Romans used this term to refer to various goddesses, such as Ceres, goddess of growing vegetation [especially, cereal] to Cybele, a nature goddess; and to other bounteous goddesses.

The symbolism is that the old school, university, or college is the bounteous fostering mother of all of its graduates.