This is the Dungeon Master's Rulebook from Dungeons & Dragons Set 1: Basic Rules, and along with the Dungeons & Dragons Players Manual 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:
Read through this booklet from front to back, just as you did the PLAYERS' MANUAL. A vast wealth of information awaits you!
In your adventures so far, you have met many fearsome monsters — goblins, skeletons, and even a Rust Monster. There are dozens of other monsters described in this booklet.
You have found treasures — coins of all types, gems, and a magical potion. There are dozens of other treasures here.
You have visited a "dungeon" — some caves near a town. There is a new dungeon in this booklet!
You will also learn how to put all these things together, make your own dungeons, run games for other players, and probably have more fun than you ever imagined.