There are several--the speaker compares the love he and his wife share to a compass and to gold. These are examples of metaphorical conceits--extended metaphors or comparisons between two items that at first seem to have absolutely nothing in common. The compass, an instrument used to draw circles, is used because the speaker is leaving his wife on a trip. She is the fixed foot who stays at home while he is the part of the compass who leaves--she leans toward him as he is gone, and when his trip is done, the circle is complete, and he returns home--together again with his wife. The circle also is a figure of speech since the comparison is to the wedding ring and the bond they have--no beginning or ending--only perfect love that completes one another.
The gold reference is to the malleable metal that can be beaten, but never broken. Gold turns into gold foil...it spreads but never breaks...much like the two of them. They leave one another, but the bond between them never breaks...just becomes thinner. He says that he does not need her eyes, hands, or lips nearby to love her still. Their love is deeper than that--it is emotional, spiritual, and intellectual. Their bond is more than just the typical physical attraction.
I love this poem--it is very romantic, though at first it doesn't seem to be. Read it again, and these things will be so clear to you.