Daenerys shows off her might by arriving on Drogon's back. This marks the first time Cersei sees a dragon, and you can basically see the gears working in her head as she thinks up a plan on how to take down the magnificent creatures — or maybe get one for herself. With the Khaleesi finally there, the parlay gets underway properly.
Cersei is quick to tell Daenerys how long they've all been waiting, and the Mother of Dragons apologises before moving on to the reason her allies are there: to broker a truce with the Lannister queen. The way Daenerys sees it, the only chance they have of surviving the army of the dead is if they focus on the North without having to worry about Cersei attacking them as well. Jon backs his queen up, but Cersei is understandably sceptical. Having never seen a White Walker for herself, she's certain this is a tactical move that will allow Daenerys to grow her army even more while Cersei pulls her men back.
Tyrion knows the only way to convince Cersei that the danger is real is to show her.