The leader adds bonuses to all units in its group and in the same hexagon. Realms begin with only one leader, called the realm character. Once attached, items adds bonuses to that unit and all other units and leaders in the same hexagon. Only a few communities belong to your realm at the start of the campaign. These are described in the Set-up Report you have received and are shown on the map in the report.
Communities provide both wealth and new units.
You may invest in it to bolster its economy, or conscript units from it, which weakens its economy. For now, all you really need to know is that communities are Good Things, and you want as many of them as you can get. You can add your own personal touch to your realm on your Turn 1 Order Sheet by changing the realm's name and banner in the space provided at the bottom. You may also change your realm character's name and class. If you decide to change your realm's name, banner, or both, certain choices aren't acceptable.