Heterogeneity in this context, refers to diversity in characteristics and content of the individual studies included in your review. Heterogeneity is not inherently good or bad, but it does affect what we can learn from the analysis.
Any kind of variability among studies in a systematic review may be termed heterogeneity.
Heterogeneity may appear in differences in population, interventions or outcomes. It can also appear in study design or results.
See this article on What is heterogeneity and is it important?
There are a variety of methods to assess heterogeneity. These include Cochran's Q Test, I-squared statistic, tau-squared, heterogeneity variance, meta-regression and forest plots.
See these articles for more information
Key concepts in clinical epidemiology: detecting and dealing with heterogeneity in meta-analysis.
For further clarification check with your supervisor or tutor.