A landmark is a type of item that points you to a location in-world. A landmark can be kept in a resident's inventory, given to another resident, put in a notecard, or given by an object through a script (if the permissions allow). If the land parcel that a resident is on allows it, a landmark can be made to that parcel through the world menu. A landmark can also be selected through the world map and a red beacon will mark that location.
To move your avatar to a landmark, right click on the landmark in your inventory, and choose "teleport." Alternately, double click on the landmark in the inventory, then push the "teleport" button on the landmark window.