A range of values you expect your test results to fall between if you were to redo the experiment.

For example, if the conversion lift is 12% ± 4%, the confidence interval would be a range of values anywhere between an 8% (12 - 4%) to a 16% (12 + 4%) conversion lift.

A confidence interval answers the question: how probable is it the conversion lift would be close to the same, if the experiment was repeated?

Typically, confidence intervals are set to 1 - the significance level.

A 95% confidence interval show the real result should be within that range 95% of the time. But may be outside that range 5% of the time.