Your home island is split up into sectors in a 3 by 3 grid and labeled 1 through 9 starting with 1 at the bottom left, in the same order as the numbers appear on the numeric part of a standard computer keyboard. You can test this by pressing 1 through 9 on your numeric keypad to jump directly to those sectors.
In many guides and in chat people will refer to these sectors so it's handy to be aware of them and how to identify which area of your map they refer to.
Sectors are also a feature of Adventure islands and work in the same way, but due to the random nature of geography the pattern of the sectors will differ island to island.
The sector grid jump is very handy for navigating the map on other devices .