A theme is an idea that runs through a text [text: Any piece of writing. More widely, a text can be anything that conveys meaning - eg, a film, tv programme, advert, website, or image. ]. A text may have one theme or many. Understanding the themes makes the text more than 'just' a text - it becomes something more significant, because we're encouraged to think more deeply about the text, to work out what lies beneath its surface.
The title of the book comes from a poem by the 18th century Scottish poet Robert Burns. It is about a mouse which carefully builds a winter nest in a wheat field, only for it to be destroyed by a ploughman. It is written in Scots dialect [dialect: The language of a particular subset of English speakers - often those living in a particular place - having its own unique diction, vocabulary, spelling and even grammar. ].
The best laid schemes o' mice an' menGang aft a-gley,An' lea'e us nought but grief an' pain,For promised joy!
(The best laid schemes of mice and menOften go wrongAnd leave us nothing but grief and pain,Instead of promised joy!)
The mouse had dreamed of a safe, warm winter and is now faced with the harsh reality of cold, loneliness and possible death. There is a parallel here with George and Lennie's joyful fantasy of a farm of their own, and its all-too-predictable destruction at the end of the story. Perhaps it is also meant to suggest to us how unpredictable our lives are, and how vulnerable to tragedy [tragedy: A type of drama in which characters undergo suffering or calamity and which usually ends with a death. A sad or catastrophic event causing suffering or death. ].