Like most of Donne's poetry, "Love's Infinitenesse" (also seen in some editions as "Lovers' Infiniteness", and "Lover's Infiniteness") is metaphysical in the sense that it takes human interaction and compares elements of it to the divine, or universal, concepts or ideas. This poem talks about the nature of love, and how love between two people compares to God's love and its infiniteness.
Donne, in the persona of the pleading lover ("If yet I have not all thy love, / Dear, I shall never have it all", lines 1-2) gets himself into a quandary with his desire for the totality of love. If he has all of his beloved's love, then he cannot get any more of it. But he desires more every day -- this sort of paradox is a standard poetic device for Donne.
Donne ends his poem pondering the conundrum of love. He wants all of his lady's love, but she cannot give it to him. If she does give it to him, there will be no more for her to give. And giving love, Donne believes, actually diminishes it in the person who loves (perhaps Donne is here musing on the inadequacy of language, for when a person "gives" something it is gone -- but that is not the case with love, or, if it is, then human love is seriously flawed). The central problem is that the lovers are temporal beings, so any participation they have in the divinity of love is limited by their temporality. Donne doesn't write standard love poetry, but this meditation on the love of another, and the difficulties in obtaining and sustaining it, is an eloquent expression of human emotion.