A "valediction" is a farewell, and this poem's farewell is a tearful one. As the poem begins, the speaker and his beloved share a final embrace before he departs on a long sea voyage.
Take a look at the enjambment in the first lines:
Let me pour forth
My tears before thy face whilst I stay here,
The broken line here suggests a catch in the speaker's voice, and it also opens up the idea that the speaker will "pour forth" more than tears. This passionate poem will reflect "on weeping," considering what lovers' tears might mean and reveal.
So long as they "stay here" in each other's arms, the speaker tells his beloved, his tears are precious. Every one of his teardrops reflects his beloved's face; it's "stamp[ed]" with her image the way a coin is stamped with the face of a monarch. "And by this mintage" (in other words, through this coin-making process), what would otherwise have been plain old tears become valuable.
This conceit—the first of many elaborate extended metaphors the speaker will explore—makes the claim that the speaker's tears are worth something because they're born of his deep feelings for his beloved. It also suggests that his beloved is a queen in his eyes, her mere image capable of making unremarkable matter into wealth.
Even beyond these ideas, this image paints a picture of infinity. If the speaker's beloved is standing where her face can reflect in all of his teardrops (already a dizzying vision: imagine all those tiny faces!), then perhaps his face is reflecting in all of her teardrops, too—and those faces are crying and reflecting, and those faces are crying and reflecting. Just as lovers' eyes endlessly mirror each other in "The Canonization," another of Donne's great love poems, the reflective, coin-like tears here create an endless shower of gold.
This tearful embrace, in other words, shows that these lovers are everything to each other: their "worth" in each other's eyes (literally!) is endless. No wonder they're crying, then. When they're apart, they feel as if they've lost everything.
The poem's shape reflects both their love and their grief. Mostly written in iambs (metrical feet with a da-DUM rhythm, as in "my tears"), the poem's lines dance between:
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Dimeter—lines with two feet, as in "For thus | they be"
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Pentameter—lines with five feet, as in "And by | this mint- | age they | are some- | thing worth"
- And closing lines of hexameter—lines with six feet, as in "So thou | and I | are no- | thing then, | when on | a di- | verse shore." (Note that the word "diverse," in Donne's 17th-century English, means "different" and is pronounced DYE-verse rather than di-VERSE.)
The short, choked-up lines of dimeter suggest the difficulty of speaking through tears; the longer lines, outpourings of thought and feeling.