"The combination of components or elements to form a connected whole"
(OED)
Forming a synthesis between various ideas is the heart of your literature review, and once done, will be the core of your research paper. As a starting point try:
• finding ideas that are common or controversial
• two or three important trends in the research
• the most influential theories.
As you read keep these questions in mind:
• does the writer make any assumptions not supported by evidence?
• what is the researcher's method? how does she gather data?
• what ideas and which researchers are frequently referred to?