Trade means to send trading vessels to other Docks (either 20 food, wood, or stone) to return with gold. The player will lose all of the resources they are trading, but the amount of gold received depends on the distance between the two Docks and the value of the goods.
Since stone is the rarest among all of the resources, it is advised to not trade it for gold and use other resources such as Food or Wood instead.
"The earliest boats were simply tied up to rocks or trees on shore to take on or drop off cargo or were physically pulled onto the beach. Later, wooden structures were built out into the water to facilitate loading and unloading. Docks were also safer for ships because ships could avoid being beached, which strained the hulls and increased leaking. When the dock was extended beyond the shallows, even larger ships could be tied up, farther improving efficiencies."