This is the Player's Manual from Dungeons & Dragons Set 1: Basic Rules, and along with the Dungeons & Dragons Dungeon Masters Rulebook makes up that boxed set. It has been separated out as a new item only because the two books from that set are sold separately in PDF format.
From the preface:
This is a game that is fun. It helps you imagine.
"As you whirl around, your sword ready, the huge, red, fire-breathing dragon swoops toward you with a ROAR!"
See? Your imagination woke up already. Now imagine: This game may be more fun than any other game you have ever played!