Gesta Danorum is a big book of stories from a long time ago, about the people who lived in Denmark. The book has lots of different stories in it, kind of like the way you might have lots of books with different stories in them.
The stories in Gesta Danorum are about the heroes and battles of the Danes, and they were originally written down by a man named Saxo. Saxo wrote the stories down a long time ago, so they are really old.
People think that the stories in Gesta Danorum might not be completely true, because they are ancient stories that have been told and changed over many years. But even if they aren't completely accurate, they give us an idea of what life was like in Denmark long long ago.
So even though Gesta Danorum might seem like a big, confusing book with lots of old stories in it, it's actually a treasure trove of information about Danish history and culture.