This line, that comes at the end of the poem, concludes the conceit, or elaborate metaphor, that Donne employs in this remarkable poem to describe the relationship of a union of souls so complete that distance cannot separate them. Note that the whole poem is addressed to the speaker's wife, in which the speaker urges her to not mourn open and visibly. After these initial recommendations, the speaker explains how they are connected so closely and how this connection will not break through death.
The speaker therefore compares their "two souls" to a pair of compasses, that are really not two separate objects, but one object made up of two parts, just as the speaker and his wife are. In an image that is remarkable for its beauty the speaker justifies his comparison to himself and his wife as a pair of compasses:
If they be two, they are two so
As stiff twin compasses are two,
Thy soul the fixed foot, makes no show
to move, but doth, if th'other do.
And though it in the centre sit,
Yet when the other far doth roam,
It leans, and hearkens after it,
And grows erect, as that comes home.
It is in this way, the speaker affirms in the quote you have identified, that the wife will stay "still" as her husband ventures off into the unknown, giving him the stability that he needs to chart his course and will keep them "joined" no matter where the husband is voyaging.