You may also want to summarize the role that dreams play in the book. Think of how the plot would be different if the concept of dreams were taken out. After you brainstorm some ideas on that, then you can analyze how central the theme of dreams is in the book.
For example, so often, people (namely adults) lose sight of dreams. Consequently, they fall into a stagnant pattern where little personal growth occurs. In the world of Lennie and George, where discrimination was rampant and times were tough as a result of the Great Depression and the Dust Bowl, dreams carried them through each day allowed them to believe that something better could be achieved.
When George Kills Lennie, he also kills a significant part of the dream. This was a sobering experience that brought George back to reality. Therefore, you can also summarize what it means to lose a dream that was held in such high esteem.
The simplest way to summarize the dream that George and Lennie have for their future is as follows: George and Lennie have been working on the farms of other people all of their lives, moving from one place to the next in search of new jobs or different lives for themselves. Their dream is to own their own farm where they can work for themselves, Lennie will have a few rabbits, and, according to George, they will "live of the fat of the land."