"In statistics, a confidence interval (CI) is an interval estimate of a population parameter. Instead of estimating the parameter by a single value, an interval likely to include the parameter is given. Thus, confidence intervals are used to indicate the reliability of an estimate. How likely the interval is to contain the parameter is determined by the confidence level or confidence coefficient. Increasing the desired confidence level will widen the confidence interval.

A confidence interval is always qualified by a particular confidence level (say, γ), usually expressed as a percentage; thus one speaks of a "95% confidence interval". The end points of the confidence interval are referred to as confidence limits. For a given estimation procedure in a given situation, the higher the value of γ, the wider the confidence interval will be." -

http://en.wikipedia.org/wiki/Confidence_level
KEY PHRASES:

"Thus, confidence intervals are used to indicate the reliability of an estimate. How likely the interval is to contain the parameter is determined by the confidence level or confidence coefficient."

74% +/- 5.15% @ 99% Confidence means:

There is a 99% confidence that this ROI is between 68.85%(-5.15%) and 79.15%(+5.15%)

I must add, this ROI will remain if all circumstances remain the same as they were when obtaining that sample. (i.e. I don't start playing significantly higher-skilled players, whereas I would start losing more than avg., or significantly worse-skilled players whereas I would start winning more then avg.)

Its easy using this calculator:

http://surveysoftware.net/sscalc.htm
BTW Ok, ROI without rake is useless for knowing my REAL avg.poker winnings. I know this, but the thread starter didn't ask this. I can post both, fine.

REAL ROI:

65.71% +/- 5.15 @ 99% Confidence