What is Confidence interval?

A confidence interval at confidence level $1 - \alpha$ (often 95 %) is an interval computed from the data by a rule that, applied to many series, contains the true value in a fraction $1 - \alpha$ of them. For the mean of $n$ results it is $\bar x \pm t\,s/\sqrt n$, where $t$ is Student’s coefficient for $n - 1$ degrees of freedom and that level.