This is a metaphysical poem and famous for its metaphysical conceits, which are odd and surprising figures of speech in which one thing is compared to another thing that is very much unlike it. These metaphysical conceits are of course presented in the context of the poem, which concerns a dying husband telling his wife to behave with quiet dignity as they part, just as virtuous and good people die without drama or display. The metaphysical imagery is used to describe their relationship as a union of souls that are so united that even death or distance cannot truly part them from each other.
The most beautiful image that the poem uses is when the speaker compares their souls to the two "feet" of the the compass, which are of course united:
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.
Imagining their souls as the feet of the compass allows the speaker and his wife to see their separation not as a "breach" but only as "an expansion" as the two feet of the compass part. Note how this powerful and poignant image is developed:
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.
This is a beautiful image that describes the way that, while separate, they still remain connected, as the centre "foot" "leans" and "hearkens" after the other foot to which it is connected. Thus it is that this poem describes the connection of a man and wife and their souls after one of them dies, and how, through their union, they never really "separate."