Sonnet 116 by William Shakespeare is about the permanence and eternal nature of love. The poet asserts that there are no "impediments," or blockages, to "the marriage of true minds"—real love. It isn't real love if it "alters" or "bends" according to circumstances. Instead, "it is an ever-fixed mark," or something that is always there as a guidepost that doesn't even get shaken by storms. "The star to every wand'ring bark" is a reference to the North Star, which always remains in the same place in the sky so that ships at sea (barks) can locate it to find their direction. Time has no effect on love, even though old age approaches and death draws near.
We see by this explanation that Shakespeare spends the first twelve lines of the poem definitively proclaiming the permanence of true love. However, he begins the thirteenth line by stating, "If this be error." If we take these few words out of context, we might interpret them as the poet indicating that he might be mistaken in his argument. We have to balance this phrase, though, in the context of the entirety of the last two lines. The poet writes that the only way it can be proven that this is error—in other words, that he can be mistaken—is if he has never written anything and no man has ever loved anyone. The poet has obviously written something; at the least, he has written this sonnet, and no one would rightfully claim that no man in the history of the world has ever loved someone.
The poet has used a negative statement to elicit a positive response. Readers are meant to consider that of course the poet has written something, and of course men have fallen in love, so everything that the poet has written about the permanence of love is not an error but is definitely fact.