The first chapter sets up the novella. First, it introduces the main characters - Lennie and George. Lennie is big and strong but mentally challenged. George is a smaller man but wise when it comes to the world. More importantly they are friends who have been through a lot. Most importantly, they have a dream. Here is what they say:
“O.K. Someday—we’re gonna get the jack together and we’re gonna have a little house and a couple of acres an’ a cow and some pigs and—"
“An’ live off the fatta the lan’,” Lennie shouted. “An’ have rabbits. Go on, George! Tell about what we’re gonna have in the garden and about the rabbits in the cages and about the rain in the winter and the stove, and how thick the cream is on the milk like you can hardly cut it. Tell about that, George.”
At this point in the story, these points might not seem significant, but as the story progresses, these points are what separate Lennie and George from everyone else.
For example, in this world, friendships do not exist. So Lennie and George's relationship is a bright spot in an abyss of loneliness and alienation. Likewise, in this world, few have dreams of a better place or life. Hence, George and Lennie's dream is revolutionary.
The first chapter sets up these themes.