The Anglo Saxon Chronicle is a written record of the Anglo Saxon conquest, settlement and development of England, from the Roman invasion to the latest entries in the 12th Century.
The chronicle was begun as a single record, which was then copied and sent out to various political/religious centres around the country. It was then updated in each centre, meaning that the later entries differ from each other, and the chronicle becomes varied depending on the viewpoint of the particular monks who were writing it!
Without this document, and Bede’s “Ecclesiastical History of the English People” in around 731, we would have a very hard time piecing together the period of history between the Roman invasion and the Norman Conquest (1066).
Although a lot of the Chronicle is written historically, and draws on previous (now lost) records, it’s thought that it from around 648 the entries are actually written “live”.