This poem is a rather simple poem where the author expresses a romantic sentiment: I have given my heart to my love, and they have given me theirs, and because of this, we are both happier. He states that this exchange was done willingly, and "there was never a better bargain driven." In other words, we wanted to give each other our hearts, and it was a wonderful bargain because we both got something wonderful that we wanted. He goes on to say that his love's heart keeps him and his love one, and his heart guides and supports his love. He says that they both cherish each other's hearts:
"He loves my heart, for once it was his own/I cherish his, because in me it bides."
The message behind this poem is that giving your heart to someone else, and getting theirs in return is a beautiful, lovely thing, and that it makes your life better. It is nice to have someone who cherishes your heart and life, and takes good care of it. I hope this explanation helps a bit. Good luck!