This rule is only applicable during the Enemy: Sail a Boat action.
After adding Settlers to an Island, the enemy now has boots on the ground. The enemy takes control if it has the most influence, gaining its Rune and increasing that god’s Fury by 1. Also, anytime the enemy adds Settlers to an Island, note if it has more Settlers (of both colors) than you. If so, remove 1 of your Settlers and return it to your mat. If the enemy now has more Influence than you on that Island, the enemy steals the Rune from you too (and increases that god’s Fury by 1). For the enemy’s Influence, all Settlers (1) and Temples (4) give their normal Influence, and all Boats (both the Chieftain and regular ones) offer 2.