This poem is a love poem. The speaker in the poem is begging his (I assume) love not to leave him. Here is a brief paraphrase of each stanza.
Will you leave me? Say no, so that you will not be responsible for making me sad.
Will you leave me when I have loved you so much for so long? Is your heart that strong? Say no!
Will you leave me when I have given you my heart and would never take it back? Say no!
And will you leave me and not have any pity? Alas for your cruelty? Say no!