Small talk is unimportant social conversation, like the small talk you make with the guy next to you on the bus — about the weather and the traffic.
You might call small talk "chitchat." It's the polite back-and-forth that occurs between people who don't know each other well and don't want to discuss anything controversial or personal. You might make small talk about the weather while you wait in line for a movie, or find yourself engaged in small talk with other dog owners at the dog park. Small talk first appeared as a term in the mid-eighteenth century, but it probably dates back to cavemen as a concept.