From YPPedia

Yer known world
Owned by all pirates
Doubloon cost: N/A

The Yer known world item is a map of the ocean, located in the charts section of the booty panel, which shows all of your memorized league points, all of the islands that you have set foot upon, and the routes and islands of charts currently in your inventory. When aboard a ship, the routes and islands of charts on the navigation table are also visible. You may view it at any time by clicking upon it and selecting "View" from the resulting menu.

While on a ship, it serves as a quick access to the View/Chart a course option. Dragging a chart on top of it will open it for viewing (or charting if available) with the map that was dropped on it highlighted in the chart view (thus serving as a quick method to chart a course on a ship).

German article: Weltkarte

Historical Notes

In release 2005-10-20 (although not reflected in the release notes), the Ye Known World item was made to be visible without having to expand the chart group first.
The Ye known world map did not always exist. Prior to appearing in the booty panel, pirates had to board a ship and use the navigation table to view a map of the ocean.

In release 2007-05-24 some additional marks were added to the maps, reflecting the flotillas of the Brigand King and his minions.

Prior to release 2008-02-05, this item was known as Ye known world.