Wikipedia
Yasna is the Avestan language name of Zoroastrianism's principal act of worship, and it is also the name of the primary liturgical collection of Avesta texts, recited during that yasna ceremony.
The function of the yasna ceremony is, very roughly described, to strengthen the orderly spiritual and material creations of Ahura Mazda against the assault of the destructive forces of Angra Mainyu. The yasna service, that is, the recitation of the Yasna texts, culminates in the apæ zaothra, the "offering to the waters." The ceremony may also be extended by recitation of the Visperad and Vendidad texts. A normal yasna ceremony, without extensions, takes about two hours when it is recited by an experienced priest.
The Yasna texts constitute 72 chapters altogether, composed at different times and by different authors. The middle chapters include of the (linguistically) oldest texts of the Zoroastrian canon. These very ancient texts, in the very archaic and linguistically difficult Old Avestan language, include the four most sacred Zoroastrian prayers, and also 17 chapters comprising the five Gathas, hymns that are considered to have been composed by Zoroaster himself. Several sections of the Yasna include exegetical comments. Yasna chapter and verse pointers are traditionally abbreviated with Y.
The Avestan language word yasna literally means 'oblation' or 'worship' . The word is linguistically (but not functionally) equivalent to Vedic Yajna. Unlike Vedic Yajna, Zoroastrian Yasna has "to do with water rather than fire."