A marketplace is an area where people buy and sell goods. In ancient times, marketplaces were usually outdoor areas where merchants would display their items for sale. Today, you can still find similar outdoor markets in many cities across the Middle East and Mediterranean region.
In New Testament times, the Greek marketplace (called an "agora") served many different purposes:
People bought and sold goods there (Mark 7:4).
Children used it as a place to play (Matthew 11:16; Luke 7:32).
Some people would spend time there with nothing to do, while others came looking for work (Matthew 20:3).
Important community events happened there, including healings (Mark 6:56).
People gathered there for discussions about community matters (Acts 17:17).
Sometimes legal trials took place there (16:19).