The first few minutes of a first date typically consist of expository chit-chat, meaning that that's when people fill each other in on the basics: where they're from, what they studied in school, and what they want to be when they grow up. It's background info.
A synonym for expository is explanatory. It might help to relate the words, as expository dialogue usually exists to give the audience an explanation of a character's previous actions. The challenge for a playwright is to communicate exposition by crafting expository dialogue that isn't boring. It's often a sign of amateur writing when the expository information stands out as just that. The experienced writer can hide the expository information inside the action and other speeches.