The tractate deals with the details of many circumstances which could arise with regards to the terumah. Thus, for example, before the terumah is separated, all the produce is tevel (untithed) and forbidden to be eaten. Terumah is considered holy, and may be eaten by priests only, as prescribed  in Leviticus 22:10, and must be guarded against becoming ritually unclean, lost or wasted, as interpreted from Numbers 18:8.
This tractate explores the subtleties of various scenarios that may arise with regard to the terumah. To illustrate, before the terumah is divided, all crops are considered unassigned and forbidden to be eaten. The terumah is viewed as sacred, and can only be consumed by priests, in accordance with Leviticus 22:10, and must be safeguarded from becoming ritually impure, lost, or wasted, as deduced from Numbers 18:8.