This is the ending line from Touchstone's speech in which he makes fun of lovers and the things they say. Here, Touchstone is saying that all people who are in love act in stupid ways. He is also implying, though, that the foolishness does not last.
First, Touchstone says that all in nature is mortal, meaning that everything that is natural will die sometime. He then says that the nature of love is to be foolish. This is what he has been trying to get across by all his silliness about kissing cows' udders and such. Finally, he is saying that the folly of love is mortal just like everything in the natural world.
Overall, then, he is saying something along the lines of "Everything in nature dies sometimes and so does the foolishness of people in love."
Touchstone says thatjust as everything that lives in nature is bound to die,similarly everyone in love is bound to make mistake that are extremely foolish.
touchstone himself has commited many foolies for his love sake such as kissing of the battler by which jane smile used to beat clothes in order to wash them,kissing of cow udders bcoz jane smile used to touch the udders while milking the coe etc...