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 Phebos, a young man living on a small island of southern Greece about five thousands years ago. Your fate is to kill a monstrous being called the Snake God, worshipped as a deity by the locals and appeased with a monthly human sacrifice.