Sir Walter Raleigh's poem "A Farewell To False Love" is an extended metaphor poem. Raleigh is simply comparing false love to many different things he feels he can compare fraudulent love to.
IN the end, Raleigh declares that any love which one deems false is "a dead root where all these fancies grew." By saying this, Raleigh is admitting that some, including himself, may try to make love into something is can never be. If the root of the love is poisoned, it cannot survive.
Basically, Raleigh is giving the reader multiple images to which they can associate a false love to. I believe that he does this so as to relate it to as many people as he possibly can. He wants to warn people about the problems associated with love gone bad.
This being said, the most important part of the poem would seem to be the title. Raleigh is admitting that one must say goodbye to a love that is poisoned or wrong. The love will never be able to change from what it is given the root of the love is dead. The love will always be dead given a healthy root, or base, is absent.