The scene in question, which seems to be a part of the hit track 'Laapata' from the film, shows Salman and Katrina posing like bride and groom and taking their wows at a Church. The couple are shown talking to each other, where their promise not to divulge the secrets of their respective nations.
Also, to our surprise, Katrina asks Salman to kiss the bride (read her) after their wows end. Enjoy the video!