The tight-gap is an Agilityshortcut found in-between Burthorpe and the Chaos Temple. An Agility level of 14 and completion of the Falador easy diary is required to squeeze past through the gap. The shortcut allows players to quickly get between Burthorpe and the Goblin Village without needing to run all the way south to the Taverley Gate.
The westernmost walkable square on the Falador side of this shortcut is considered members only, and unwalkable in free-to-play worlds due to a map-tile border. This prevents free-to-play players from unlocking Principality, the Members-only music track that plays in that square.