What is the allegorical significance of the poem "Crossing the Bar" by Alfred, Lord Tennyson?

Allegory is defined as any work of literature that, upon interpretation, reveals a deeper—and often moral or political—meaning. In order to answer your question, it’s important to understand that some scholars also use the term "allegory" when referring to a complex metaphor.
Alfred, Lord Tennyson’s famous poem is one such metaphor. In the text, the speaker compares sailing out into an unknown sea with the journey into the afterlife.
The speaker embarks on his trip at sunset, which is used to indicate it is the end of the speaker’s life—in other words, he has reached old age. He hopes that no one will be sad over his gradual drift toward death, which he describes as “darkness.” The bar represents the luminal space between the earthly realm and a spiritual one, and once the speaker transverses this boundary, he will have left life behind him.
The pilot that the speaker mentions at the end of the poem could suggest a guide who leads the speaker to his final destination, or it could be an angel or other spiritual being.