Fleecing in fantasy football is the act of taking advantage of a bad general manager by conducting a lopsided trade. If you fleece a general manager, it strengthens your team greatly as you get a big return while not giving up much.
Often times a lopsided trade happens because one general manager doesn't know enough about football to realize the trade is unfair.
The best way to make sure a trade is fair is to look at the statistics that your fantasy website provides about players. The most important stats include points and % owned. A player who has many points or is projected for a lot of points is a very valuable player since the whole point of fantasy football is scoring points. Remember that the % owned is the percent of leagues the player is rostered in, as opposed to being a free agent. The very best players are near 100% owned, and you should be wary of trading them. It is rarely a good idea to trade a 100% owned player for a player that is owned in a much lower percentage of leagues.
If you're not sure about a trade, you can always ask a friend if it seems like a good idea. Just don't ask the one who is making the trade with you!