The lovers in this poem encounter their own shadows. The shadows are a metaphor for the way we hide parts of ourselves from a beloved. We often do so before the love has fully blossomed, at which point we feel secure. At that moment, the high point of love, we reveal ourselves fully, without shadow, to the other person.
Donne's speaker compares this fullest point of love to noon, the time of day when all shadows disappear. He says that, like noon, it can pass very quickly. Lovers can start to disguise things or keep secrets from each other, and this can cause the fullest love to fade.
Lovers need to be mindful that love will "decay" if they aren't fully open and honest with each other. True love, the speaker implies, takes hard work and a complete commitment to transparency. Otherwise the shadows will deepen and deepen until the light of love darkens into night.
Because of the difficulty of staying in the "noon" of love, the poem can seem pessimistic. True love is depicted as fleeting, like noontime, lasting only for a brief moment. Yet by talking to his beloved frankly and realistically about the need to strive to keep love at the high point of "noon," the speaker increases the possibility that their love will flourish.