The I2 statistic is used to assess heterogeneity.

In case of I2, we usually define what means high, moderate or low. For example, if you define that I2 > 75% is considered as substantial heterogeneity and I2 of your meta-analysis is more than 75%, that means considerable heterogeneity is present. For instance, if the objective of your meta-analysis is to find out the presence of 'X' in patients compared to controls; in this scenario high heterogeneity means some study results are turning towards patients (some studies observed significant presence of X in patients) and some study results are turning towards controls (some studies observed significant presence of X in controls). kind of high deviation from no effect line. If this study's I2 is low (i.e., 40%), you would see that the results are turning towards a particular group (more studies towards either patients or controls, non-heterogenous).