What does the phrase 'the best laid plans Of Mice and Men often go awry' mean?

The Words are from "To a Mouse, on Turning Her Up in Her Nest with the Plough" written by Robert Burns in 1785. The original text is

The best-laid schemes o' mice an' men

Gang aft agley,

(Scots)

"of Mice and Men" is the name of John Steinbeck's novel which comes from this poem.