A Valediction Forbidding Mourning is the title of a song by John Donne.

Right from the title you can understand what the song isbout:  the deals with farewell. 
the  song  is organized into nine quatrains, that are stanzas made up of four lines each.
It is in alternate rhyme: it also creates a musical effect. It can be considered an argumentative song because the poet explains why, in his opinion, friends should not weep when virtuous men pass mildly away and a lover should not cry when his or her beloved leaves for a while.
 In the first stanza the poet describes the transition between life and death for virtuous men because they are virtuous and then they aren't afraid of death. Those are able to face their death without fear but unfortunately other men are sinful. This stanza describes how the " virtuous men" pass away so gently that some of their friends doubt that they have passed away, indeed.

The poet makes a parallelism between men who die and the separation of two lovers (probably his wife and himself). The only difference is that a dead man can not come back home as does a lover. Donne uses two hyperboles, tear-floods and sigh-tempests to invite his wife not to become sad because his departure should not be considered a tragedy. As a matter of fact he gives a positive connotation even to death as underlined by the adjective mildly.
 In the second stanza the poet elevates his love from earthly love to spiritual love , in fact he adds that his love is sacred and more elevated than the love of ordinary people: "Twere profanation of our joys to tell the laity our love". In the third stanza Donne creates a metaphor of an earthquake to compare it to his love; he describes how a moving of the' earth may bring hurts and fear but his love has more meaning to it, being profound: "..trepidation of the spheres..."

In stanza four Donne resumes the theme of love. The Dull lovers are human beings who build their love only on body contact. In this case absence will dissolve the lovers' bond as does that dissolve men's bodies. he describes other lovers as "sublunary" because this love is imperfect.

Quatrains five and six provide a description of poet's and his wife's love. In the fifth stanza the poet mentions three parts of body because they are the most important body parts of sense of the ordinary love. He also says that his love is refined and these body parts aren't so important to feel in love. In quatrain six  he describes how pure his love really is and he compares it  with  the purity of gold. , the gold is considered a noble metal and so difficult to destroy as love

The last three quatrains form a metaphor the poet uses a compass to describe the love of the two lovers. .: only when one is fix in the centre (in this case his wife), can the other draw a perfect circle and makes me end where I begun. Donne's wife is his firm point, a point of reference.