Effect size is a number that shows how large a difference or relationship is in a study, rather than only whether it is likely to be real.
A study can find a difference that is statistically significant and still tiny. Effect size tells you whether the difference is big enough to matter in daily life.
A common measure is Cohen's d, which expresses the difference between two groups in standard deviations. By Cohen's own guidelines, around 0.2 is small, 0.5 is medium and 0.8 is large.
When you read that a treatment 'works', look for the effect size. It tells you how much it helped on average, which is more useful than knowing that it helped at all.