Synthesis refers to combining separate elements to create a whole. When reading through your sources (peer reviewed journal articles, books, research studies, white papers etc.) you will pay attention to relationships between the studies, between groups in the studies, and look for any pattterns, similarities or differences. Pay attention to methodologies, unexplored themes, and things that may represent a "gap" in the literature. These "gaps" will be things you will want to be sure to identify in your literature review.
Galvan, J. L. (2006). Writing literature reviews (3rd ed.). Glendale, CA: Pyrczak Publishing