At the beginning of the sonnet, the speaker proclaims that "love is not love / Which alters when it alteration finds." In other words, true love, according to the speaker, is not love which alters with changing circumstances. If the person you love became ill, for example, then you would still love them just as much as you did before, if the love was true love.
The speaker then proclaims that love "is an ever-fixed mark" and "the star to every wand'ring bark." The star referenced here is the north star, which always maintains the same position in the sky and which was therefore used by sailors in the seventeenth century as a reference point by which they could navigate their ships. The "wand'ring bark" ostensibly references a ship, the word "bark" being a synecdoche for a ship, but metaphorically the "bark" represents a lover. The meaning of the metaphor is that love will guide a lover through times of darkness just like the north star will guide sailors in the night. True love is thus not only constant and unaltering, but is also a source of guidance.
The speaker confirms this idea about true love at the end of the sonnet. He says that "love alters not with brief hours and weeks." This means that true love does not alter with time. It does not become less with each passing day, but rather will always retain its initial intensity and passion. The speaker is so certain that he is correct about the nature of true love that in the final lines he declares that if he is wrong then "no man ever lov'd." In other words, if the speaker's idea about love is incorrect, then, in his opinion, whatever love is can not be worthy of the name.