The religious Jurisprudence (Fiqh) is the religious rulings that are derived from the religious sources of proof: the Holy Book and the Sunnah of the Prophet and the Imams (peace be upon them). These ruling were known during the beginning of Islam by direct referral as the Prophet and the Imams (peace be upon them) used to explain them to people in simple and understandable ways. As time passed, and people became farther away from the era of the legislation, views differed in understanding the religious texts and the conclusions they derived from them. Fiqh changed from being simple and understandable by the lay people to a specific field of knowledge that required experience to determine the religious rulings. From such a development, the Principles of Fiqh were developed and become a separate field. It discusses the general principles that the jurist requires in the process of derivation.
In short, Usool Al-Fiqh is the study of the general rules that the process of derivation depends upon.