Synthesizing sources provides a big picture viewpoint that describes how the ideas or themes in the sources are connected. Not all sources have to agree or provide the same viewpoint, but they all provide perspectives on a central theme or idea. This differs from summarizing sources, which describes the ideas or themes in those individual sources.
A summary just restates the key findings or arguments of others.
A synthesis shows how the ideas of multiple sources are related and incorporates your own critical interpretation of your research findings.