John Donne was a metaphysical poet, which means going beyond or transcending the physical or material world. In terms of understanding his context or time period, Donne was a clergyman who believed in the afterlife and in a spiritual realm beyond this world. He was noted for sleeping in a coffin so that he would remember he was going to die. In this poem he leans into the idea of "negative theology" of Augustine and others, the idea that the Supreme Being can be defined not by what it is but what it is not. Donne applies this to romantic love.
In the first stanza of this poem, addressing his beloved, the narrator scorns the typical lover (and/or poet), who focuses merely on the tangible qualities of the beloved: he says he never stooped so low in describing his love as to use merely physical terms, such as preying on "eye, lip, cheek." The word "prey" implies an animalistic kind of love that focuses on the carnal and wants to devour it. These lovers, like animals, "soar no higher" than to admire the looks or virtue (which we can define as both physical chastity and good behavior) of the beloved. In other words, these lovers do not think about the unseen, spiritual aspect of love.
In the second stanza the narrator expresses the impossibility of expressing this "more" or spiritual dimension of love in words. He rejects reducing love to mere physical attributes, writing:
To all, which all love, I say no.
He asks anyone who can explain what this intangible aspect of love is to decipher it for him. Yet he ends by saying that even without words, he cannot "miss" or leave out this aspect of his love, this "nothing" part of love, for it is where his comfort is. He does not speed past it.
This is fascinating poem because we all know there is a part of love that cannot be explained through the looks and behavior of the beloved. This is the mystery at the heart of love. We do not all love the most objectively beautiful or virtuous person in the world, and as Shakespeare points out in A Midsummer's Night Dream, once we fall in love, we see that "nothing" in a lover that Donne is groping to describe, that spiritual, inexplicable aspect, as when (though comic) Titania sees beauty in Bottom with his ass's head. What we love in another is, in part, beyond words, beyond what can be explained.