Your alma mater is your old school, college or university. It's generally used as a positive term, implying reverence and loyalty for the nurturing qualities of the institution.
Alma mater comes from two Latin words meaning "nourishing or bountiful mother." Originally it was used as a term by the ancient Romans to describe their Goddesses, but by the early eighteenth century in Britain it had come to refer to one's university. Because of this historical background, there's a suggestion about alma mater that it refers to a prestigious place of learning — though that association has largely disappeared.