A
Trade Route is the process of loading a trade good on one
Market and delivering it to another
Market.
Trade Routes are commonly used as objectives on
Merchant Quests. In fact, you'll only be able to load
trade goods at a
Market if you're taking a trade quest at that moment.
Each
Market deals with a specific trade good. For example, you may have a
Market that deals with
Supplies. You may load up
Supplies to your
Fleet on that
Market, but the
Market won't accept
Supplies as a trade route.
There are also some
Quests that require more complex
Trade Routes: you may have to trade on more than
one trade good, or even do it on a deadline (example:
Complete 10 trade routes using Tools or Supplies in 24 hours).