After being rejected by both his brother and Bronn, Tyrion is in a dark place. To his surprise, Oberyn Martell comes to pay him a visit. He tells Tyrion of Cersei's visitation and their conversation about Myrcella, which he notes was obviously a ploy on her part to influence his judgement towards Tyrion. Oberyn then tells the story of when he visited Casterly Rock with Elia when they were young. They were desperate to see Tyrion, as tales of his grotesqueries were far spread. They were ultimately disappointed when he turned out to be just a baby. Even back then Cersei wanted Tyrion dead, which only reaffirms what he already knew. This segues into why Oberyn really came to see him: he will be Tyrion's champion so he can get revenge on the Mountain for what he did to Elia and her children. It feels so good to see Tyrion's spirits lifted when Oberyn comes to the rescue. As if Oberyn wasn't an awesome character already, him rescuing Tyrion from sure despair only makes him more likeable. His story about his first laying eyes on baby Tyrion, and the look Tyrion had when he realised just how long Cersei has waited for this, is a powerful moment. It also provides a nice anecdote about how the world sees Tyrion versus how he really is; a parallel to the end of the last episode. Oberyn's declaration sets up what is sure to be a clash of titans in the next episode; a fight book readers know is one of the best action sequences of the series.
