What does the poet mean when he says, "And having nothing, yet hath all"?
In the first stanza, the speaker notes that the happiest person is one who is not guided, directed, or oppressed by another person. The happy person focuses on honesty and truth. In the second stanza, the happy person is not afraid of death. He is focused solely on himself. He does not have ties to the world nor does he concern himself with fame. In the third and fourth stanzas, the speaker adds that the happy person does not envy anyone and is not concerned with rumors or flatterers. He prays for God's grace, not for gifts. He has no need or desire for material possessions (gifts).
In the last stanza, the speaker says the happy person is "freed from servile bands." This means that the happy person is not a slave to another person, a desire, or compulsion to possess something or be dependent upon another person or object. In short, the happiest person is one who is independent in every way. With no concern for material possessions and no concern for the need to possess things in general, the happy person does not own or possess anything. He is not tied to possessions and therefore has "all" that he needs: independence. He is neither reaching for nor being pulled by some other object, person, or desire.