Effect size

An effect size can be as plain as the difference between two means or two proportions, or it can be standardized by dividing that difference by a standard deviation. It answers how much, while a p-value only answers whether there is evidence of any difference at all. For example, if one group averages 78 and another averages 75 with a standard deviation of 10, the raw effect is 3 points and the standardized effect is $3/10 = 0.30$. AP Statistics does not use the phrase effect size; it asks you to report the difference itself with a confidence interval for it, which is the same idea in plain units.