It's a textual adventure (some graphic embellish it, but is purely decorative and has no role in the game).
You give commands usually composed by a verb plus a noun (e.g. "go north", "examine desk", "drink potion") and are told what happens ("you cannot go in that direction", "there is a pen on the desk", "you feel reinvigorated"). You have to explore the locations, collect items, use them when appropriate and in general imagine how to overcome the various problems in order to accomplish the final goal of the adventure.
You are Nigel Stevenson, a young guy in modern times who followed the indications of a legend and travelled to an alternative (fantasy) world. The princess of such world had been kidnapped by an evil wizard, but you managed to defeat him and free her.
You are now going to help the king of neighbouring realm, where a prized amulet has been stolen and must be recovered from a scarcely known and dangerous area.