"Love's not Time's fool" captures the controlling metaphor of this Shakespearean sonnet as all other lines reinforce this sentiment: "Love is not love" if it changes with the corrosive power of time, "Love alters not with his brief hours and weeks," "it is an ever-fixed mark."
There is, however, a flaw in the poet's logic. For, in the final couplet, he challenges his reader's to disprove his argument as though it is logically sound. But, the first lines establish a condition to this reasoning:
Let me not to the marriage of true minds/Admit impediments.
So, if the reader does not accept this condition and does, in fact, admit impediments, or obstacles, to love, then the argument of the poet is not unequivocably reasonable, and may not be "an ever-fixed mark."
I agree with the previous poster. I want to add, though, that you should probably only discuss this part of the poem in terms of the complete thought that contains it:
Love's not Time's fool, though rosy lips and cheeks / Within his bending sickle's compass come.
This thought might be paraphrased as follows: Love is not deceived simply because time erases our youthful looks (which I take to mean our physical attractiveness). I agree with the statement, too, because I think that physical attraction and love aren't exactly the same thing.
The first source given below may be making an unsupportable claim about the significance of the capital letter in "Time". Early Modern English is full of capital letters in words that we don't capitalize today.
The line that you cite argues that love is eternal and is not to be subordinated to time.
As long as we're talking about love within the lifetime of the lover, I agree (at least at this point in my life). Some people say the poem is talking even about love after death, and I'm not sure about that.
But for love within a lifetime, sure. I met my wife when we were both 17 and we're about to turn 40. We've been married 20 years next June. I can't pretend that love feels exactly the same as it did when we first fell in love, but I sure don't feel any less in love than I did back then.
So based on my experience so far, I agree with the sentiment.