Reading lists are OPML documents that point to RSS feeds, like most of the OPML documents you find, but instead of subscribing to each feed in the document, the reader or aggregator subscribes to the OPML document itself.
F. Abel, N. Henze, E. Herder, and D. Krause. User Modeling, Adaptation, and Personalization, volume 6075 of Lecture Notes in Computer Science, Springer Berlin Heidelberg, (2010)