The answer to this question comes just a line before, when we are told who "his" refers to. Often a lot of the meaning of lines can be understood if you take them in context and read immediately before and after to make sure you understand what is being referred to. Have a look at these lines:
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come.
In these lines therefore, the speaker of this sonnet alludes to the medieval image of time as the grim reaper, who cuts off life with the sweep of a sickle. Notice the point that the speaker is making - although youth is clearly under the power of Time as we all age, yet Love is not "Time's fool," meaning Love does not alter or change as we grow old. This of course ties in to the overall theme of this sonnet, which is the permanence of love.