I would like to have an explanation of what exactly these three things are so that I can explain them to my friends: (1) transubstantiation, (2) supererogation, and (3) the temporal power of the pope.
(1) Transubstantiation is the transformation in the Eucharist of the substance of bread and wine into the substance of the body, blood, soul, and divinity of Christ. This transformation of substance does not affect the appearances of the elements, which continue to appear to be bread and wine. The substance changes; the appearances do not.
(2) Supererogation is doing more than what is required. For example, in 1 Corinthians 7 Paul counsels the Corinthians that he who marries does well, but he who does not marry does better (7:38). Paul thus indicates that Christians have the freedom to marry but that it is possible for them to do even better than this. Embracing a celibate life for the cause of Christ would be a case of supererogation—doing something more than what was required, since one is free to marry.
(3) The temporal power of the pope is that power which he exercises as a governmental rather than a religious leader. For example, the pope is the head of the Vatican City state. When he acts in this capacity, he is exercising temporal power as the head of a government. In former centuries the pope often had larger domains than just Vatican City state (which is tiny) and so used to have more temporal power than he does now.