Trade routes are a means of making money by going from station to station and buying/selling goods. It's most beneficial when using a transport ship(ship with large cargo hold). 

Trade routes can be simple back and forths between a single set of stations, or a massive network between a great number of stations. 

As you trade between stations, the values of the items you're trading may go down(inflation), depending on the number of people trading and the amount of stock available.