A correlation is a notable similarity in direction of two variables. One part of fantasy football directly affects the other, for example increased passing = increased catches.
Offense in football that score a lot of total points tend to have good fantasy options on the team. Drafting multiple players from one good team is called stacking, and a way to capitalize on the correlation between offensive rank and fantasy points.
Similarly, correlation exists in the way a team runs its offense. Teams with more pass attempts per game tend to put up better quarterback and wide receiver stats. While teams with more rush attempts have better running back fantasy production.
This can also be done in season when deciding what players to start each week. If your players are playing against a bad defense, then starting them is making use of the correlation opponent defensive rank and fantasy points.
Below is an image showing how short left completion percentage has a positive correlation with fantasy points per game.
Noticing and taking advantage of correlation puts your team at a major advantage. By learning what two variables affect one another you can acquire players with higher value more consistently. Rather than somewhat randomly drafting players you can set up a true strategy and stick with it.
So, although it may seem like a lot of work or math to find correlations, all you really need to do is look at the good players and see if there's a connection between them.
The previously listed examples of correlation are a great place to start, but if you have the time before your draft look at how other stats affect one another. By doing so you can uncover a correlation that'll give you a leg up over the rest of your league!