A Valediction, Forbidding Mourning

John Donne

As virtuous men pass mildly away,
     And whisper to their souls to go,
Whilst some of their sad friends do say
     The breath goes now, and some say, No:

So let us melt, and make no noise,
     No tear-floods, nor sigh-tempests move;
profanation: desecration. The speaker considers their love to be sacred, not earthly (profane). laity: lay people, as distinct from the clergy.
'Twere profanation of our joys
     To tell the laity our love.

Moving of th' earth: i.e., earthquakes.
Moving of th' earth brings harms and fears,
     Men reckon what it did, and meant;
trepidation of the spheres: a now obsolete theory of oscillation in the precession of the equinoxes. innocent: not destructive, and not a portent of evils to come.
But trepidation of the spheres,
     Though greater far, is innocent.

sublunary: earthly; literally, "below the moon."
Dull sublunary lovers' love
admit: allow entrance into, as in the phrase, "admissions office." those things which elemented it: those things which constituted its elements; i.e., the "eyes, lips, and hands" mentioned in the next stanza.
     (Whose soul is sense) cannot admit
Absence, because it doth remove
     Those things which elemented it.

But we by a love so much refined,
     That our selves know not what it is,
Inter-assured of the mind,
     Care less, eyes, lips, and hands to miss.

Our two souls therefore, which are one,
     Though I must go, endure not yet
A breach, but an expansion,
     Like gold to airy thinness beat.

If they be two, they are two so
stiff twin compasses: i.e., a draftsman's compass. This singular object has a plural name, like "a pair of pants."
     As stiff twin compasses are two;
Thy soul, the fixed foot, makes no show
     To move, but doth, if the other do.

And though it in the center sit,
     Yet when the other far doth roam,
It leans and hearkens after it,
erect: upright and fully human, as distinct from beasts which creep on all fours. —Donne is quite willing and able to employ sexual witticisms, but I don't think he is doing so here.
     And grows erect, as that comes home.

Such wilt thou be to me, who must,
     Like th' other foot, obliquely run;
just: perfect.
Thy firmness makes my circle just,
     And makes me end where I begun.