The ß glyph is a lowercase letter than represents a ligature between a long s and a round s, and is still used today in (some versions of) German. Its uppercase equivalent is two characters instead of one: SS.
It was apparently also once used in just the same way English, but I cannot find just exactly when or where. Was it used in manuscript only, or in printed books? During what time period would this have run? If in print, was it done only in blackletter faces in English, or was it also done in the less German-looking ones?
Somewhat related is the question What animal is a “weefil”?.