Synthesis is making connections between your sources. Identifying the similarities, differences, relationships, and patterns of your sources will help make your argument stronger and easier for your readers to follow. Synthesizing is not the same as summarizing. Synthesis is an important part of scholarly writing - learning how to synthesize will elevate your academic work.
Summarizing is restating the information in one or more sources without offering new perspectives.
In his new book, Bruce Wayne dives into the the history of the Justice League of America. He argues that Batman is the true leader and founder of the team (Wayne 1939). Clark Kent, in his book on the Justice League, argues that Superman is and has always been the true leader of the team. His book further expands on how Superman served a central role in the founding of the group (Kent 1938). Both books argue that the league was created by one man's actions. Other researchers have argued that the team came about organically in response to the growing organization of villains (Jordan 1960). In another book, Gordon (1996) criticizes Kent and Wayne for leaving the efforts of female heroes out of the narrative.
Synthesizing brings together information from several sources with your own analysis to offer a new perspective on a topic.
The founding of the Justice League of America has long been attributed to the will of one individual (Wayne 1939, Kent 1938). Scholars supporting this view argue that the world's superheroes recognized the need for collective action on a global scale, with Batman and Superman suggested as primary instigators (Wayne 1939, Kent 1938, Prince 1942).
The issue with this theory is that it leaves out the motivations of lesser known heroes and and the growing pressures placed on heroes by the increase in villains seen in the 1920s. The increase in global villain activity necessitated a more organized response on the part of superheroes, to prevent the end of peaceful society as we know it (Jordan 1960). Heroes began responding to crime in a more organized way with the formation of local agencies by the early 1930s, predating the formalization of the hero network we now call the Justice League by at least 25 years. This thesis is bolstered by accounts of informal team-ups in 1930-32 gathered by Wayne (1939).