With tag mode you can trade items, maps and information to other Dragon Quest players when you are nearby.
To start using Tag Mode you need to goto the Quester's Rest and talk to Erinn and then choose the "Canvas for guests" option.
At this point your DS will start searching for other players nearby and connect if there are any, you can also close the lid of your DS and it will continue to canvas for guests while you are out and about.
There are treasure maps that will show you hidden areas with monsters and treasures. After you complete one of these you can trade these maps with other players via Tag Mode. You will also be able to trade some rare items as well.