Low Win Rates When a champion has a low win rate it indicates that over the course of the sample size in a range in time, this specific champion has historically underperformed.

What this DOES explain: Whatever people generally do with this champion may not be optimal; This champion's strength may be lower than the majority of other champions; and This champion's core strengths may be heavily outperformed by other, more popular champions in the current meta. What it does NOT explain: The potential overall strength of the champion; The potential strength of a specific rune/mastery/skill/item combination; and The strength of a champion in a different and shifting meta-game (champion selection, strategies, etc.) This means that if any of the aforementioned changes or becomes apparent to more of the population, then this champion could experience a sudden increase in win rate and I'll likely experience a sudden increase in complaints in champion placement. (Unless I had the foresight to place them high to begin with, which means I've been getting complaints the entire time - See Wukong) High Win Rates These champions have historically won more often than other champions during the times that they're picked.

What this DOES explain: The current most popular item/skill/mastery/runes on that champion appear to be working; This champion's combination of damage, utility, and survivability seem to be line with the majority of other champions currently popular; and The times when this champion is selected are favorable to that champion (may be a counterpick, but if the pick percentage is high enough, this seems unlikely.) What this does NOT explain: That this champion is a good champion to pick in every scenario; That a top 10 win rate champion is necessarily better than a top 20; and That this champion will win their lane match-up more often then not.

Read more: http://www.nerfplz.com/2014/06/win-rates-what-do-they-mean.html#ixzz3MIR3EBwG