From the rules: "In the early 1600s the Iroquois Five Nations initiated a number of wars to gain control of the beaver trade. Each of 3 to 5 players represents a clan (Wolf, Bear, Turtle, Eagle, or Deer) within the Iroquois Confederation during this time and attempts to gain control of Confederation policy and the beaver trade."
There are two main elements that players must deal with - influencing the five nations to follow your clan and attacking the enemy tribes.
Each nation grants a different benefit when it follows your leadership. Control of a nation is determined with an area control/influence mechanism. If one is willing to surrender their influence during a council turn, more beaver tokens (victory points) are earned.
When attacking neighboring tribes, other clans may join yours in the attack. Their help will improve the chances of victory, but means the spoils will be shared. Success is determined by a simple superiority of numbers after the defending tribe has had a chance to eliminate the attackers. Success earns the survivors a share of that tribe's beaver furs.