The prologue of the Prose Edda has very little to do with what heathens believe, and is generally ignored as a quaint fantasy. To the compiler, however, it was the portion that allowed the rest to exist, as it provided the rationalization that allowed the Pagan mythology to be transmitted and studied by Christian scholars. It's primarily concerned with the concept of natural religion, as discussed previously. In it, "the author makes it clear that his view of the pagan religion is that it was a rational but misguided groping towards the truth" (Sturluson 1987, xviii).
That the prologue was written by Snorri is by no means certain. "There is indeed great embarrassment about whether the Prologue is by Snorri or not, whether it makes its author Christian or not, whether it shows faith in the heathen gods or not, and whether it has any function in the Edda. Scholars hesitate endlessly between diverse positions" (DePins 771). It contains hints of Latin scholarship, which some use as an argument that Snorri could not possibly have been the author. It's one of the cases where the copies presented in the different existent manuscripts are not the same, and there has been much debate over who wrote the extra material. Most translators stick to the Codex Regius version, though Rasmus B. Anderson's translation contains the additional material present in Codex Wormiamus. Like the rest of the Edda, it is divided into chapters which would make a direct comparison between translations easier were it not for the fact that the chapters are occasionally ordered differently based on different original manuscripts, and that many translators do not indicate where the chapters begin and end.
The prologue opens with an abbreviated retelling of Genesis from the Christian Bible. It begins with Adam, proceeds rapidly to Noah, and spends most of its time on the loss of connection with the Christian god. Despite that loss of connection, the Christian god gives man signs of his existence through various natural features and functions, describing how all things are connected. At this point, there is a split between the Codex Regis and the Codex Wormiamus, as the latter then goes on to tell how the world is split between Noah's sons, and the tower of Babel is built. Zoroaster is mentioned, as is the founding of Babylon, and the beginning of the worship of other gods. Then the two codices converge again.
The next chapter describes how the world is arranged, naming the three continents known at that time. The next describes the founding of Troy, and the structure that it builds. It emphasizes the number twelve, separating things into twelve kingdoms, rulers, and languages with one overking. This is in order to align with the twelve tribes of Israel, and the twelve apostles. The author is beginning to build his case for how his "natural religion" parallels the Christian version.
At this point, the codices diverge again, with Wormiamus again having added material, which euhemeristically describes how the Greek gods came to be, beginning with Saturn. This continues for four chapters (each chapter being only a paragraph long). Finally, the two codices converge again at Troy, where the euhemerism continues with the descent from Priam to Thor, through Thor's descendants to Odin. The reason this is done is in order to make the argument that the Norse mythology was actually descended from the Greek, which for whatever reason had been excused from the usual idea the Christian church put forth that the gods were actually demons deceiving mankind. Thus, the author attempts to extend that exception to the gods of his own ancestors. If the gods were actually humans who had been elevated to divinity by other men, then they couldn't be demons, so the study of mythology becomes the study of history, which was acceptable under the church's regime.
The prologue then continues through Odin's migration to the northern lands, and how his children become rulers over the various kingdoms that they encounter. Along the way, it names his sons as assigned to rule various places, as are the names of other deities such as Freyr, heroic figures such as the Völsungs, and the ancient names of the royal families such as the Ynglings. Finally, Odin reaches Sweden, where king Gylfi grants him land, and then he proceeds into Norway as well. The language that was brought with Odin, presumably descended from Troy as well, becomes the language of the North. Odin's family then intermarries with the people there, the people become settled, and the region becomes Norse.
The prologue of the Prose Edda was probably the last piece written of the fo6ur sections. It's unknown whether it was written by Snorri, another enthusiast, or simply added by a scribe. That it was altered by scribes is almost a certainty. It presents the origins of the Norse pantheon in such a way that immunizes it from church censure. Far from being an amusing anecdote, as it's treated by modern Heathens, it actually contains the logic which allows the rest to exist. Without this prologue, even more of our mythology may have been lost.
de Pins, Cyril. "The Fantastical Theology of Snorri Sturluson: A Reading of the Prologue of Snorra Edda" in In The Fantastic in Old Norse/Icelandic Literature: Preprint Papers of the 13th International Saga Conference, Durham and York 6th-12th August 2006, I-II, ed. John McKinnell, David Ashurst and Donata Kick (Durham: Centre for Medieval and Renaissance Studies, 2006). Available at http://sagaconference.org/SC13/SC13.html Consulted 15 March 2023
Dronke, Ursula & Peter. "The Prologue of the Prose Edda: Explorations of a Latin Background" in Pétersson, E. G., Kristjánsson, J. Sjötíu Ritgerðir, vol. 1. Reykjavík, 1977
Sturluson, Snorri translated by Anthony Faulkes Edda. Everyman, London, 1987