It's simple; the game has a remarkable complicated text parser. Just type and go, it even understands full sentences. You're not limited to stuff like "take leaflet", you could "Take the leaflet from the mailbox then drop it on the floor."
Think of it as an interactive novel, where the goal is to acquire lots of different treasures scattered around. Infocom, the company that made the Zork series, released dozens of text based adventure games throughout the '80s. Some fantasy, some sci-fi, a couple murder mysteries, many that defy easy categorization.
(Quite possibly the most difficult Infocom game ever released. You're the emergency manager of a futuristic city, kept in suspended animation until the advent of a disaster. You're immobile and cut-off, and can only interact with the world through six robotic assistants).
If anyone finds an online version of Starcross I'd love to know