The cestui que use is the person for whose benefit the trust is created. The cestui que trust is the person entitled to an equitable, as opposed to a legal, estate. Thus, if land is granted to the use of A in trust for B, B is cestui que trust, and A trustee, or use. The term, principally owing to its cumbersome nature, has been virtually superseded in modern law by that of "beneficiary", and general law of trusts.
The cestui que use and trust were rooted in medieval law, and became a legal method to avoid the feudal (medieval) incidents (payments) to an overlord, while leaving the land for the use of another, who owed nothing to the lord. The law of cestui que tended to defer jurisdiction to courts of equity as opposed to common law courts. The cestui que was often utilized by persons who might be absent from the kingdom for an extended time (as on a Crusade, or a business adventure), and who held tenancy to the land, and owed feudal incidents to a lord. The land could be left for the use of a third party, who did not owe the incidents to the lord.
This legal status was also invented to circumvent the Statute of Mortmain. That statute was intended to end the relatively common practice of leaving real property to the Church at the time of the owner's death. Since the Church never died, the land never left the "dead hand" ("Mortmain" or Church). An alternative explanation of "mortmain" was that an owner from generations earlier was still dictating land use years after death, by leaving it to the Church. Hence the term "dead hand." Before the Statute of Mortmain, large amounts of land were bequeathed to the Church, which never relinquished it. This was in contradistinction to normal lands which could be inherited in a family line or revert to a lord or the Crown upon death of the tenant. Church land had been a source of contention between the Crown and the Church for centuries. Cestui que use allowed religious orders to inhabit land, while the title resided with a corporation of lawyers or other entities, who nominally had no relation to the Church.
Read more about Cestui Que: History in German and Roman Law, Medieval Invention, Typical Medieval Patterns, Cestui Que As A Method of Fraud, Henry VII, Henry VIII, United States Case Law of Recovery From Disseisee in Cestui Que, United States Rule Against Perpetuities