Big monuments, harbours and the junkyard are territories. Stay in one alone long enough and you capture it for your group (team, clan or solo). If two groups are inside, it is contested and the capture pauses.
The holding group regularly gets scrap for every online member plus random territory buffs; without an enemy takeover a territory becomes free again after a few hours. Held territories give leaderboard points. See the status in the "Leaderboards" tab (with territory map and capture history per territory) and on screen as soon as you enter a territory. If your territory is attacked, you get a notice.