Usul al-fiqh is the discipline that studies where law comes from and how rulings are derived, not the rulings themselves, which are fiqh, but the theory governing their production. It covers the sources and their ranking, the classification of commands and prohibitions, the general and the particular, the literal and the figurative, abrogation, conflict between indicators, and the qualifications a person must have before deriving anything at all.
Its formative statement is the Risala of al-Shafi'i (d. 820), which argued for a definite hierarchy and, decisively, for the authority of the Prophet's sunna as established by sound hadith rather than by the living practice of a city. Later works built it into a technical science: al-Juwayni and then al-Ghazali's Mustasfa on the Shafi'i side, al-Sarakhsi and al-Dabbusi on the Hanafi, and, at the end of the classical period, al-Shatibi's Muwafaqat, which reframed the whole enterprise around the purposes the law is taken to serve. Twelver Shia usul developed in parallel, with its own formative literature from al-Tusi onward.