The speaker begins by comparing the minutes experienced by a human during his or her lifetime to the waves of the sea. Each one follows immediately after the one that goes before it, but they all are headed in one direction: the shore.
Then the speaker thinks of a different parallel for human life—the sun. The sun rises in the east, full of light. Then, it slowly makes its way up the sky to its position at high noon. But then, out of nowhere, "eclipses" come and blot it out. The speaker ends this second section by talking about how destructive time is. You know, because it kills us and all.
The third section of the poem focuses in detail on time, who gets personified as a destructive force laying everything to waste, Attila the Hun-style. And finally, the couplet of the sonnet—its last two lines—tries to turn back the tide (so to speak) against time a little bit. How? By reminding us that at the very least, this poem will last, and continue praising "thy worth" (whose worth? We'll find out) through the centuries.
Up to this point in the poem, we haven't been given any indication that the speaker is speaking to anyone in particular. The last line of the poem thus surprises us by revealing that everything we've heard up to this point is actually part of a love poem. And not just any love poem, either. Shakespeare was right—Sonnet 60 is one for the ages.