A presupposition is a belief that is assumed (presupposed) from the very beginning that governs the way we think.
Presuppositional apologetics is a method of arguing for the truth of the Bible that analyzes the validity of presuppositions. In contrast, evidential apologetics uses a shared presupposition between Christians and non-Christians, and then argues using proofs and evidence that depend on this shared presupposition. While there is a place for evidential arguments, Christians should recognize the problems and limitations with these kinds of arguments.
Epistemology is the study of knowledge and how it is possible. An ultimate presupposition starts at the very beginning and provides an epistemological theory for how knowledge is possible.
Presuppositional apologetics argues that the only presupposition that can successfully explain how knowledge is possible is the presupposition that the Bible is God's revelation.