The line "burning notes of fire" in the given sentence is a conceit used to symbolize the intensity and passion that poets bring to their works about freedom. The word "burning" suggests an intense heat, which can represent the fervor with which poets write about freedom. The word "fire" also implies passion, inspiration, and creativity, which are all qualities that poets may associate with the pursuit of freedom. Overall, this phrase conveys the idea that poets use their art to express their deep-seated emotions and beliefs about freedom, inspiring others to do the same.