Here is an explanation of the sonnet:
Let me not to the marriage of true minds Admit impediments. Love is not love Which alters when it alteration finds, Or bends with the remover to remove:
The first quatrain of the sonnet argues that true love should not be impeded or influenced by outside forces. The speaker insists that true love is steadfast and unchanging and does not alter when faced with adversity or change.
O no! it is an ever-fixed mark That looks on tempests and is never shaken; It is the star to every wandering bark, Whose worth's unknown, although his height be taken.
The second quatrain compares true love to a "fixed mark" or a guiding star that can withstand the storms of life. The speaker states that true love is a constant presence in our lives, like a star that guides a ship through stormy seas. The true value of love cannot be measured, but it remains a powerful force in our lives.
Love's not Time's fool, though rosy lips and cheeks Within his bending sickle's compass come; Love alters not with his brief hours and weeks, But bears it out even to the edge of doom.
In the third quatrain, the speaker argues that true love is not subject to time and its effects on the physical world. While beauty may fade with time, true love endures even until the end of life. The speaker argues that true love is not temporary or fleeting but remains constant through all of life's trials and tribulations.
If this be error and upon me proved, I never writ, nor no man ever loved.
In the final couplet, the speaker declares that if his definition of true love is proven wrong, then he has never written anything of worth, and no man has ever truly loved.
Overall, Sonnet 116 is a powerful statement about the nature of true love. The speaker argues that true love is steadfast and unchanging, a guiding force that endures through time and adversity. The sonnet emphasizes the importance of love as a fundamental part of the human experience, and as a force that remains constant and enduring even in the face of life's challenges.