What does the expression “a rift in the lute” mean and Where does it come from?

It is the poet, Alfred Lord Tennyson, to whom we are indebted for the saying “a rift in the lute”, and he expressed its meaning in the lines from The Idylls of the King (Merlin and Vivien, 1870) in which it appeared:

Faith and unfaith can ne’er be equal powers:
Unfaith in aught is want of faith in all.
It is the little rift within the lute,
That by and by will make the music mute,
And ever widening slowly silence all.