Confidence interval
A confidence interval is a range that plausibly contains a true value, given the data observed so far.
In plain English
It puts honest error bars on a performance figure. On small samples the interval is wide enough to include both a strong edge and no edge at all, which is exactly the useful information a point estimate hides.
Example
An ROI of 5 percent over 40 bets might carry an interval spanning clearly negative to clearly positive.
Why it matters
A single number invites more confidence than the data supports. The interval is the part that tells you how much to trust it.