The tone of a text (including poetry) is basically the emotion that comes from the text as it relates to the author's emotional message. The emotional message conveyed will largely depend on the author's purpose. Just like emotions or attitudes, tones can be positive, negative or neutral. In poetry tone is transmitted through the choice of words and images created.
If we consider Wordsworth's "I wandered lonely as a cloud" for example, we find the tone is very positive and could therefore be described with such adjectives as inspired or ebullient.
Adjectives are always used to describe tone and here are some more examples: sarcastic, sincere, patriotic, regretful, solemn, rousing, religious, outraged, excited, sentimental, humourous bitter, melancholic, angry and serious (to name but a few).