A way to express how big an effect is, on a common scale.
The Standardised Mean Difference expresses the size of a difference between two groups relative to the variability in the data, so results measured on different scales can be compared and pooled in meta-analysis. Rough rules of thumb call about 0.2 small, 0.5 moderate and 0.8 large, though context matters. It answers 'how big', separate from whether a result is statistically significant.