William Butler Yeats's poem "Words" is about the relationship, or perhaps more accurately the clash, between life and art. We learn to use words in order to communicate. For many people, it remains the primary purpose of words to convey such messages as "Please pass the marmalade" or "I'll be home late tonight."
Writers, however, want use words to do more than this: to impart something of the human experience and to arrange the words in such a way as to give them beauty and power. The poet, despite his own bitterness, which he thinks his beloved does not understand, has done his best to explain it to her, using the words that are the tools of his trade. If she still does not understand, at least he has done all he can "to make it plain."
In the third stanza, the poet seems most confident in his abilities. He has become a proficient poet. Words obey his call, and so he has made her understand anything by sheer poetical power. However, he finishes by reflecting that, if he had as much power over people as he does over words, he "might have thrown poor words away" and "been content to live."
This is open to interpretation, but it seems to me to mean that the poet has spent his life mastering words at the expense of personal relationships. If his beloved had truly understood and accepted him, he might have lived with her in contentment, instead of continuing to try to express himself through poetry. His use of the word "poor" to describe words would suggest that they are an inadequate substitute for a fulfilled life of understanding with another person.