Conflict is basically a problem or obsticale in a story.
In Of Mice and Men
The conflict in the book is that Lennie is just holding George back. Everytime Lennie gets in trouble its affecting George's life. This includes preventing him from settling down, getting a wife, and starting a family. An example of what happens when Lennie gets into trouble (extreme example), would be when Lennie accidentally killed his puppy and Curley's wife. Since he did that, George had to make the desicion to kill him to keep him from being punish. This was very hard for George to do because Lennie was pretty much his best friend.