This is one of Shakespeare's serious statements on the nature of ideal true love, which begins with a play on the words from a traditional marriage service:
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.
It is not coincidental that the standard marriage service contains the phrase "if any of you know cause or just impediment why these two. . . ." Shakespeare elevates the sentiments in this sonnet to the level of one of the most solemn parts of the marriage ceremony. In addition, he makes an important statement about men and women: the marriage he refers to is the "marriage of true minds," not the conjoining of a superior to an inferior. Further, in the ideal marriage, based on true love, one partner does not attempt to alter the other for convenient purposes.
In the second quatrain, Shakespeare compares true love to a lighthouse, "that looks on tempests and is never shaken," and to a star, "the guiding north star," that provides a fixed point of reference for a ship lost at sea. In other words, true love, ideal love, provides a fixed reference point for the two lovers to guide them back to the ideal should they lose their way.
The third quatrain argues that true love is not susceptible to time:
Love's not time's fool, though rosy lips and cheecks/Within his bending sickle's compass come./Love alters not with his brief hours and weeks,/But bears it out ev'n to the edge of doom.
The effect of time on true love, in Shakespeare's view, is negligible--although physical attractions many fade over time, such things have no effect on true love, which stays absolutely constant and unchanged until the end of time.
Shakespeare ends the sonnet with a couplet that essentially says if he is proven wrong about the nature of true love, then all of his writing can be ignored and, more importantly, he admits that he has never truly loved.
The power of this sonnet is that it is so absolute, so positive, about the nature of love. Shakespeare doesn't say "true love is like this;" rather, he says, "this is what true love is." There is no doubt, no vagueness, no equivocating, about the nature of love--true love defies all that is thrown in its way.