Synthesis means to combine a number of different pieces into a whole. Synthesis is about concisely summarizing and linking different sources in order to review the literature on a topic, make recommendations, and connect your practice to the research.
Synthesis usually goes together with analysis because you break down a concept/idea into its important parts/points (analysis), so you can draw useful conclusions or make decisions about the topic or problem (synthesis).
In the field of Humanities, synthesis (in the case of philosophy) is the end product of dialectic (i.e. a thesis) and is considered as a higher process compared to analysis.