In the movie Avengers: Age of Ultron Steve Rogers said to Bruce Banner
I have seen her flirt, upclose
It was about Black Widow. What does that mean? Is that some dialogue theatrics or is there any reference there?
Yes, In Captain America: Winter Soldier, there's a scene where Natasha kisses cap to avoid getting caught by bad guys. Also check this question and I believe he was referring to that situation.
Here's the scene.