Historia Regum Britanniae is a highly mythologized, and possibly fictional, history of Britain prior to the Anglo-Saxon conquest. Published in 1136, the work covers 2,000 years between the Fall of Troy to the reign of Cadwallader the last non-Saxon king. Geoffrey claims to have translated an ancient historical book written in the Welsh language, but it is more likely that he invented the Alternate History genre. It may be wrong to judge the book by historiographical standards though because it could be argued that in the 12th century mythology and history were not separate topics.
In Geoffrey's account Britain is first settled by Trojan refugees led by Brutus, the grandson of Aeneas who according to Virgil founded the city of Rome. The heroic early British have to battle giants in order to claim the land. Britain rapidly becomes a power equal to Rome and the Romans acknowledge them as cousins as they are fellow descendants of Troy. The British never loose a battle unless base treachery is involved. Their enemies are cowardly traitorous barbarians worthy of extermination, and the Saxons are the lowest of all.
Geoffrey's work was very popular in Medieval times and was the source of the story of King Lear and his daughters, and contains the earliest known account of King Arthur. It is unlikely that Geoffrey invented Arthur, but his account is so over the top it is hard to credit that there might be any shred of actual history in it. Arthur manages to conquer pretty much all of northern and western Europe before defeating the equally fictional Roman Emperor Lucius Hiberius.
Fans of Arthurian legend may be disappointed by Geoffrey's account as it contains none of the high romance the later stories. There is no Lancelot, Parsifal, Tristan or Galahad, and Guinevere is present in name only. Arthur's only retainers are Bedivere, Kay, Gawain, and his treacherous nephew Mordred. Merlin is a major character but oddly he and Arthur never meet in the story. Merlin's involvement is entirely with Arthur's father Uther Pendragon. I say Merlin is a character, but there are no real characters in the book. Like most Medieval histories there are long lists of kings, some are noble and some are wicked, but no one is portrayed with any actual human foibles or characteristics.
If you are a fan of Medieval literature then you will probably enjoy Geoffrey's history. It can be a gripping read in places, although his descriptions of battle seems like the author had only a vague notion of how wars are conducted. For Arthurian fans I would say Geoffrey is mostly for completists and not nearly as entertaining as Chretien de Troyes or Thomas Mallory.