In Shakespeare's poem, "Let Me Not to the Marriage of True Minds" the speaker says love "is a star to every wandering bark." The star is functioning as...

In this sonnet, the star is functioning as something that is constant, something a wanderer could look to in order to get his bearings and know where he is. Without the aid of modern navigational instruments and technology, the traveler on the sea would have to count on the stars to provide something constant by which to steer. The speaker of the poem says, "It is the star [...]," and the it refers directly to love. Thus, the star in the poem is a metaphor for love. A metaphor is a comparison of two unalike things where the poet says that one thing is another. This particular metaphor comparing love to a guiding star suggests that love can provide the same kind of constant reassurance and guidance that the stars can.

In Shakespeare's Sonnet 116, the speaker compares love to "a star to every wandering bark." This is a metaphor in which love is compared to the North Star or a constellation that is used by sailors to guide their ships, or "barks." In Shakespeare's time, sailors would often guide their boats at night by looking at the North Star, as it did not change direction in the sky. The North Star provided them with guidance through the dark seas in the days before modern navigational tools.
In this sense, the speaker is saying that love is a force that can guide people when they are lost and that provides them with direction when they are at sea (metaphorically speaking). Therefore, the star in this sonnet functions as a metaphor that stands for the ways in which love can provide people with guidance and direction.