Activity coefficients for chemicals in a mixture are an indicator of what the concentration of that chemical will be in a vapor of the mixture.

In an ethanol/water mixture that is less than 95% ethanol, the activity coefficient for ethanol is higher than water. Therefore the concentration of ethanol in the vapor from the liquid will be higher than in the liquid itself.

At 95% ethanol, the activity coefficients are the same, meaning that the vapor is also 95%. This is called an azeotrope.

Above 95% ethanol, the activity coefficient for water is higher than ethanol, so the fraction of water will be higher in the vapor than in the liquid.