What does it mean ?

Parting is such sorrow - Romeo and Juliet

Poets have long been fascinated by the concept of dialectical emotions--that to be truly happy, one must first be deeply sad; to enjoy the delights of food, one must first experience hunger.

So it is with Juliet's situation. Saying goodbye to Romeo triggers deep sadness, but that sadness also reminds her of her love for him, and for this reason it is sweet.

Saying goodbye also initiates her anticipation of seeing him again, which gives that emotion a pleasant tingle.

Remember, too, that for Shakespeare "sweet" meant not necessary "pleasing," but also "not offensive," which tempers the romantic meaning of the phrase just a bit.