Effect size is a quantitative measure that helps to understand the strength of a relationship or the magnitude of a difference between groups in research. It provides a standardized way to compare results across different studies, making it easier to interpret the practical significance of findings, rather than just relying on p-values. Common types of effect size include Cohen's d, which measures the difference between two means, and Pearson's r, which assesses the strength of a correlation between two variables. By using effect size, researchers can better communicate the importance of their results to both scientific and general audiences.