"Yes, and" is an exercise wherein you relate your character to the one posted before you. So it should be a story about when/where the two characters interacted and what went on. This also helps establish how the characters relate to each other a bit.

They don't have to have a nicey-pooh relationship; Tony Barnes and Flint White, in the last RP, for example, had a brush with each other because Flint was investigating Tony on behalf of the courts during a spree of lycanthrope killings and found out about Tony's Vietnam war history. It was a false accusation that created bad blood.