Someone who has territory under their sleeve is the leader. They're the boss of all this territory because they say so, and because they shoved it all into their big sleeves.
This is a very formal word that is often used in political context, particularly when referring to the leader of a faction or political party.
This is a jukugo word that uses the on'yomi readings of the kanji. You haven't learned the on'yomi reading for 袖, so here's a mnemonic to help you remember:
It's always hard to fill a leaders' shoes (しゅう). Leaders have big responsibilities and power. Just big shoes to fill! Picture these big ol' leader's shoes, like in a literal sense. Not only do they have big responsibilities, leaders have big shoes as well.
What a boss says is absolute.
That politician is the leader of the party.
These days, the word "leader" tends to be used in the political sense.