The barsom, called baresman in the older Avestan form of the word, is a bundle of thin twigs, traditionally cut from specific plants such as tamarisk or pomegranate, bound together and held by a Zoroastrian priest throughout the Yasna and other major liturgical ceremonies, the bundle bound near its base with a strip of date palm leaf called the aiwyaonghana. Mills' translation of the Yasna and related Avestan liturgical texts documents the barsom's central place in the ceremony's prescribed structure, held and ritually manipulated by the priest at specific points across a long liturgy, functioning as one of the fixed, indispensable ritual implements of Zoroastrian worship rather than as an optional or merely decorative accessory to it.
The plant material itself carries theological weight within Zoroastrian tradition, which holds vegetation as one of several good creations of Ahura Mazda deserving of ritual honour, so the barsom's presence at the heart of the liturgy expresses, in physical form, the wider Zoroastrian concern with maintaining and honouring the purity and goodness of the created natural world across the whole of its ritual life. A worn or damaged barsom is not simply discarded casually but replaced according to its own prescribed procedure, reflecting the same general principle found across Zoroastrian ritual practice that a sacred implement's handling, from first preparation to eventual retirement, follows deliberate, prescribed steps rather than ordinary convenience.