Effect size measures a) the magnitude of the difference between two or more groups. It quantifies the size of the difference rather than just indicating whether that difference is statistically significant. By providing a standardized measure, effect size allows researchers to assess the practical significance of their findings, rather than relying solely on p-values.
For example, in a clinical trial comparing two treatments, knowing whether one treatment is statistically significantly better than the other is important, but understanding how much better it is in a practical sense (the effect size) is equally crucial for determining real-world relevance.