The title comes from a famous quote from Robert Burns' poem To A Mouse:
The best laid schemes o' mice an' men
gang aft a-gley (go oft awry)
In the novel, George and Lennie have grandiose plans of buying their own place, raising chickens and rabbits, and in Lennie's famous phrase, he got to take care of the rabbits. Their plan seems almost to become reality when they discover a small plot of land for sale, and take in a partner who will help with the down payment. Sadly, it is not to be. Lennie kills Curley's wife, albeit by accident, and George knows immediately that their plans will never come to fruition. In an act of self sacrifice, he kills Lennie rather than see him captured and abused for something he does not understand.
Thus George and Lennie's "best laid plans" go awry. Hence the title of the novel.
The title, Of Mice and Men, is a reference to a poem written in 1786 by Scottish poet Robert Burns entitled, "To a Mouse". The poem is an apology from a man who inadvertently plows a mouse's nest under. Specifically these two lines from the poem apply to Steinbeck's book:
The best-laid schemes o' mice an' men
Gang aft agley, (often go awry)
Paraphrased this means that even the most carefully prepared plans may go wrong. George and Lennie have dreams of someday owning their own ranch but these aspirations careen hopelessly off track.
It is about mainly 2 people one is a follower and the other is a leader. The follower is the mouse and the leader is the man.