Academic writing is designed to allow the reader to form a synthesis, or overall mental picture, of the topic which it covers. Keeping the whole of a complicated argument in your mind at once is very difficult, and academic writing aims to help the reader do this by having a very clearly defined structure and by offering the reader very obvious cues.
In terms of the literal meaning of the words, "synthesis" is the opposite of "analysis", since "synthesis" means fitting together a number of different things into a whole, whereas "analysis" means breaking something up into its parts. In terms of what you need to do to write university essays, however, it makes more sense to see "synthesis" and "analysis" as mental activities that go together. You can only put together a new whole in your essay if you have spent time defining the problem by breaking it down into its parts, just as in arithmetic you can only divide fractions if you have found the lowest common denominator.