So in FoE, there are goods, which are somewhat rare resources and are traded on the market. There are certain trade ratios which many players call "fair trade", which is usually 1 same age good: 1 same age good, or 2 goods of an age lower:1 good of the age above it. Now, some people will be desperate and provide much better trade ratios for you in order to get a quick trade. So if you see anybody who is offering, for example, 15 lumber for 10 stone, that would be a favorable trade, since a trade ratio that is "fair trade" is 10 lumber for 10 stone. You're getting 5 extra lumber in this trade than what most players would offer, which makes this trade favorable for you.