Zork is a text adventure where you enter text commands and then the game responds in text - to describe the surroundings, highlight items or places of interest, etc. Like, the game says “You are in a forest,” and you type “go south,” and the game updates to tell you what’s in your new location. It’s like the AI text adventures you see now (which are based on Zork-style games), but with every command and response hand-coded.
The Grues are monsters that eat you in Zork. “You were eaten by a grue” is a common bad ending.