Ok, I'll try very hand to explain this in the simplest terms possible.
Getting the definitions of the way, standard deviation is a measure of our friend (and enemy), variance.
It means that on any other day, if you held the same cards but the community cards came out differently for all of your hands in the database:
You could easily lose 15.6327 BB/100 more from what you actually won because of horrible community cards coming up (bad luck).
Conversely, you could easily have won 15.6327 BBs more than what you have aactually won, due to great community cards coming up (good luck).
In fact, your true winrate for all your hands on average (average good and bad community cards) is
WR (+ and -) 15.6327 BBs/100
where WR is the winrate you happened to get for all your sessions.
i.e. PokerTracker says you won 3.2 BB/100 for all your sessions, then WR = 3.2
So your true winrate would be:
= 3.2 (+ and -) 15.6327 BBs/100
So the minimum true winrate is (3.2BB/100) - (15.6327BB/100) = -12.4327 BB/100
Your maximum true winrate is (3.2BB/100) + (15.6327BB/100) = 18.8327BB/100
The range of your true winrate is between -12.4327BB/100 and 18.8327BB/100.
You put as an interval like this (-12.4327, 18.8327). That's a huge range, because it doesn't even tell you if you're a winning or losing player yet!
Also, this huge range isn't even that accurate. We can only say that your true winrate is between (-12.4327, 18.8327 with a 68.4% level of confidence. We're 68.4% sure that your winrate is anywhere between there. So we're not even that sure. However we can say that your winrate is in (-28.0654, 34.4654) with a 95% level of confidence.
The theory behind the previous paragraph is something like this:
A standard deviation is used to show the variance (we've all heard that term, right?) of statistics of the same category, i.e. the statistic of hit avg. for all Yankee players.
In short, the sample mean (+ or -) 1 s.d. gives you a 68.4% probability that any randomly selected Yankee player has a hit avg. between the interval (mean-1sd, mean+1sd).
For more info that has minimal jargon, try
this.
jaxup, reply as soon as you can so I can touch this up for other posters who are in the same boat as you.