As virtuous men pass mildly away,
And whisper to their souls to go (1-2)
It's the beginning of the love poem and this guy is talking about dead guys? Really? Donne is being sneaky. He is starting the poem with the exact moment when the body and soul split. So we continue through the whole poem with that crucial distinction in our minds.
Dull, sublunary lovers' love (13)
One more time, sublunary means "beneath the moon." But more broadly, he is trapping all other loves to the earth with all its dirt and worms and icky physicality. If their love is down there, then his love exists among the deeply spiritual stars.
But we by a love so much refined
That we ourselves know not what it is (17-18)
Donne is a really smart guy. So, if he says he doesn't know something, it isn't just a head scratcher—it's unknowable. He likes thinking of love like that. It's not some earthly thing you can't point at and figure out; it's a big, spiritual mystery.
Our two souls therefore, which are one, (21)
There's something about this description that reminds us of the Holy Trinity. Can two souls truly be unified in similar fashion? Donne sure seems to think so.