And though it in the center sit, Yet, when the other far doth roam, It leans, and hearkens after it, And grows erect, as that comes home.
These lines go some way toward making us feel better that Donne is more than a chauvinist. The whole stanza is dedicated to the "fix'd foot," to her. He doesn't blab on about all the great things he's going to do while he's away. He focuses on the importance of that fixed foot.
The first two lines here keep us turning and turning with more contrasting, paradoxical language: "though" and "yet." When we boil it down, though, it's not too bad. Even though the fixed foot is stuck there in the center, it follows after the roaming foot by leaning.
The leaning is personified with the word "hearkens." This calls to mind the title of the poem, which forbids mourning at his departure. We imagine Donne's wife (or her heart) longing outward toward her husband across the channel. That personification is mirrored in line thirty-two—the other foot, like Donne, will come "home" again.
The word "erect" will inevitably always elicit a snicker when the poem gets read out loud in class. (And honestly, most of the time, Donne's wordplay is hinting in that direction. He's got some really naughty poems out there.) In this case, though, considering we are talking about his wife, it's probably a safe bet that we are just talking about how she will (like the compass foot) stand tall and firm again when her husband is on his way safely home.