The marketplace in Manor Lords is one of the game's trickier mechanics. You'll likely first encounter this building when you receive a message from one of your families asking for space to set up a market stall where they can distribute their goods. If you want your populace to have plenty of fuel and food early on, you should build one sharpish.
As you assign more families to chop wood, forage food, and produce items, you'll start to see more stalls appearing on the market providing their goods to your subjects...