What does book title Of Mice and Men mean?

The title "Of Mice and Men" is actually a line from the poem, "To a Mouse" written by Robert Burns in 1785. The poem is about how a mouse's den was destroyed, leaving the mouse vulnerable. In the last two stanzas, the poem states that the mouse is not alone in its troubles and that humans go through the same thing. The theme of the poem is that most things don't go as planned for most people, including Lennie and George. While most people think that the title was chosen because of the symbolism of mice in the book, it was actually chosen because the themes of the poem and Steinbeck's work are relatively the same. So in full, to understand the meaning of the title you have to understand where it came from. look at the stanza in the poem that includes "of mice and men" and you will understand the meaning.