Ancient Egypt was a land of mysteries, and one was the collection of prayers, spells, incantations, and instructions known as the Book of the Dead.
埃及《亡灵书》：通往永生之旅 For one thing, the Book of the Dead was not in fact a book, since the Egyptians didn't have book binding. It was a papyrus scroll that was placed in the sarcophagus with the deceased, and texts from it were also often written on the walls of the tomb and inside the sarcophagus.
For another, the Egyptians themselves didn't call the collection the Book of the Dead. That was a name given by German Egyptologist Karl Richard Lepsius in 1842 when he published some of the texts.
The Egyptians believed that when a person died, he still had a lot of work to do. What? Are you kidding? He had to make a long journey to paradise and encounter a series of ordeals on the way. The Book of the Dead contained the prayers and spells—and instructions for how to use them—that he would need on the trip; a sort of afterlife travel guide.
The Egyptians believed that if the person made it to the afterlife, his soul could come out of the tomb during the day and, in essence, live again. He could enjoy all the same things he had in life. The Egyptians had a complicated view of the soul. Every person had a ba, which was like his personality, and a ka, which was his life force. The ba was dependent on the physical body, and that's why Egyptians were mummified. After all the proper funeral rituals and after the person made it to paradise, the ba and the ka were united to become the akh, or the completed soul. The akh was a blessed spirit that could then live the good life for eternity.