This is a great question, and it just so happens to be one of Shakespeare's most famous sonnets of all time!
In short, the moral of the sonnet (as with most of his sonnets) is summed up in the concluding couplet:
"And yet, by heaven, I think my love as rare /
As any she belied with false compare."
In layman's terms, Shakespeare believes that his mistress is as rare and beautiful as anything the world has ever seen. He thinks that she is so beautiful, in fact, that it is foolish to try to compare her "false"ly to anything else (as so many typical poems so often do).
The sonnet is a wonderfully self-aware reflection both on beauty on the very craft of love poetry itself. Simply stated, Shakespeare is acutely aware of the fact that love poems are often riddled with overused cliches and implausible comparisons, and so he writes this poem as a reaction to those flowery Petrarchan sonnets that had so long dominated the industry. Rather than spending an entire poem building his lover up by likening her to a string of impossible comparisons, Shakespeare instead chooses to tear his mistress down somewhat, setting up each alternating line to extol the beauty of a natural marvel only to reiterate just how unimpressive the natural features of his mistress may well indeed be when gazed on in such a laughable comparison.