Regions of tiles on the map are assigned to an area. Each area can be entered and exited by movable atoms as they move across tiles. As you cross a /turf and enter a different area, /turf/proc/Entered() is called on the turf and /atom/proc/Entered() is called on the area.
/area
name = "Somewhere Unknown"
/area/Entered(atom/movable/entering_object) // called when something (an object or mob) moves into this area
entering_object << "You've just entered [name]" // we send the name of the area to the object. if this is not a mob, the operation is ignored
/area/entrance_way
name = "The Entrance"
