In "A Valediction: Forbidding Mourning" the speaker encourages his lover to handle their upcoming separation bravely. The first six lines set up a comparison between the calm, dignified death of men who have lived good lives and the similarly dignified behavior which the speaker is hoping to see from his love.
The poem is structured as a set of conceits, or complex images that create logical equivalences between objects or situations that may, at first glance, seem very unlike one another. Metaphysical poets like John Donne are known for their extensive use of such comparisons, and since they are often expressed in long, grammatically complex sentences that stretch over several lines or even stanzas of the poem, they can be difficult to parse. When you encounter such conceits, try using strong punctuation marks, such as colons or semi-colons, to mark the divisions of thought; they often help to break the poetic argument into more manageable steps.
The poem's first stanza is comprised of one long clause ending in a colon, which signals the reader that the first stanza is one complete unit of thought. Indeed, in this case, it provides us with the first part of the comparison: a man close to death. Because he has been "virtuous" in life, he has nothing to fear in death and can face it so calmly and quietly that the friends around his bedside cannot even tell precisely when his breathing stops. The second stanza, beginning with "so," introduces the second part of the comparison. Just as the good man dies calmly, the speaker urges his lover to "make no noise" when they have to part. He wants her to avoid "sigh-tempests" and other melodramatic expressions of sorrow that the "laity" might use so that their love can be set apart as superior.
The conceit suggests a similarity between death and the parting of the lovers. While physical death results in permanent separation, the lovers' separation will only be temporary; but by comparing their situation to one that has even more serious consequences, the speaker aims to both conciliate and counsel his lover. The virtuous death brings religious considerations into the poem which are further strengthened by Donne's use of the words "laity" and "profanation" in lines 7-8. We are in sacred territory here, and by using this terminology to talk about their relationship, the speaker elevates it to the level of the holy. Having imparted this sacred character to their love, the speaker is able to compare himself and his lover to priests or special initiates whose superior understanding places them far above the "laity;" just as the virtuous man's quiet acceptance of death is a mark of his spiritual maturity, their ability to part from one another without vulgar expressions of overwrought sentimentality will be an outward sign of their love's transcendence.