While screen time for the Greyjoys was minimal last season, their impact on the war is significant. On one hand, we have Euron Greyjoy doing the bidding of Cersei Lannister. On the other hand, we have Theon Greyjoy, whose fight has been about forgiveness and redemption. As with all things in Westeros, let's start with the bad news.
Having double-crossed Daenerys, Euron Greyjoy sails the Iron Fleet to King's Landing and meets with Cersei, the new Queen of the Seven Kingdoms. She proposes an alliance, which Euron quickly counters with a marriage proposal. Cersei declines, but Euron promises a gift that will change her mind. He eventually returns with Ellaria and Tyene, the women responsible for the death of Cersei's daughter, Myrcella. The move ingratiates him to the queen, who orders him to lead his fleet west to Casterly Rock, where he attacks and destroys the remainders of Daenerys's ships.
Meanwhile, Theon Greyjoy has a heart to heart with Jon Snow. He explains that he's admired how Jon always seems to know the right thing to do and that his own missteps have occurred because of the push and pull he feels between his Greyjoy heritage and his Stark upbringing. Jon doesn't see this as an adequate reason for Theon's betrayal but forgives him and reminds Theon that he can be both a Greyjoy and a Stark. Encouraged, Theon confides his need to save his sister, Yara, from the duplicitous clutches of their brother, Euron, rather than join the pending fight in the North. Jon encourages Theon to follow his heart.