From the Wikipedia article:
Sensemayá is based on Afro-Cuban religious cults, preserved in the cabildos, self-organized social clubs for the African slaves. African religions were transmitted from generation to generation. These religions, which had a similar but not identical structure, were known as Lucumi or Regla de Ocha if they derived from the Yoruba, Palo from Central Africa, Vodú from Haiti, and so on. In this poem we meet an adept known as the mayombero. He is knowledgeable in the area of herbal medicine, as well as being the leader of rituals. In Sensemayá, the mayombero leads a ritual which offers the sacrifice of a snake to a god, perhaps Babalu Aye. This god, popularized as Babalu in the United States by Desi Arnaz, is the Afro-Cuban spirit who has the power to heal, or spread pestilence. One of the main motives in Sensemayá is based on this word mayombero. This chant "mayombe, bombe mayombé", is an example of Guillén's use of repetition, derived from an actual ceremony.
It is not infrequently described as a setting of Guillen's poem (hence the confusion in some quarters that it was originally a vocal work): not in the traditional sense however -
it is purely instrumental - and "the music is drawn from and related to the structure and semantic content of the poem". In other words, there are rhythmic musical motifs which are derived from a syllabic setting of the text. This can be seen in this analysis which is in Spanish - don't let that put you off - just scroll about 5/8s of the way into the text and you'll see musical examples of motifs with words from the text underneath.
http://repository.eafit.edu.co/bitstream/handle/10784/1456/JuanGabriel_AlarcónCarreńo_2013.pdf?sequence=1&isAllowed=y
