Motif supplements the theme of a piece of work. It is a recurring idea or action or image or word or symbol in a piece of work, which through repetition aids the narrative by projecting the central theme of the work.
Motifs in Shakespeare's sonnets support several themes including love, beauty, time, ageing, jealousy and so on. In Shakespeare's sonnets addressed to a young man, one of the motifs is the repeated imagery and suggestions of the passing time and the declining youthfulness and beauty of the young man. For example:
Lines 1-4 from Sonnet I
From fairest creatures we desire increase,
That thereby beauty's rose might never die,
But as the riper should by time decease,
His tender heir might bear his memory:
Lines 1-4 from Sonnet II
When forty winters shall besiege thy brow,
And dig deep trenches in thy beauty's field,
Thy youth's proud livery so gazed on now,
Will be a totter'd weed of small worth held:
Lines 1-4 from Sonnet III
Look in thy glass and tell the face thou viewest
Now is the time that face should form another;
Whose fresh repair if now thou not renewest,
Thou dost beguile the world, unbless some mother.
... and so on, motifs can be observed in his other sonnets, too and on other themes, too.