That the best laid plans of mice and men often turn out ugly (Rpobert Burns 1785), i.e. That all our plans, no matter who or what we are, often fail, and sometimes in tragic ways.
Someone has probably point out to you that the title comes from Robbie Burns' poem "To a Mouse" (If you haven't read it, it's here: http://www.worldburnsclub.com/poems/translations/554.htm ), where...
It's an allusion to a line from the poem "To A Mouse" by Robert Burns: "The best schemes o' mice an' men gang aft agley". This means that the best laid plans of mice and men often go astray. This is...