Yupa is basically sacrificial post in Hindu rituals but it has more symbol and meaning in various Hindu scriptures. As per Aitareya Brahmana (II.1), yupa, the sacrificial post, is described as a bestower of food, prosperity, happiness, brilliance, splendor and dwelling in heaven.
The term yupa is derived from the root ‘yu’, which means to bind with unadi suffix ‘pa’ (kuyubhyam a – Unadi III.27). Bhattoji Dikshita explain it more clearly in Siddhanta Kaumudi – yupa is a sacrificial stake to which the item of sacrifice is attached.
Satapatha Brahmana (I.6.2.1) states that Bhagavan effaced the traces of sacrifice with the sacrificial post so it is called Yupa. Aitareya Brahmana (II.1) narrates as a legend on the erection of yupa which says that God obstructed men and seers from tracking the world of heaven by means of sacrificial post.
Sanskrit lexicons record the term yupa as a synonym of a consecrated pillar (samskrta stambha). According to etymologists, the God of herbs, Vanaspati is the sacrificial post yupa (Nirkuta, VIII.17; also Rig Veda I.13.11).
As per Pranagnihotra Upanishad (IV.1), the sound om is compared with the sacrificial post in self sacrifice. The heart of sacrifice is also compared with yupa in Mahanarayana Upanishad (XXV.1).
Different types of sacrificial posts made of different wood with different lengths to size and different victims fastened to those yupas are described elaborately in Vedic literature. Yupa is to be made of khadira (acacia catechu) for soma sacrifice. Other woods used are bilva, palasa, rauhitaka or bibhitaka and deodar.
In Jyotistoma sacrifice, one should tie either one or eleven items to sacrificial posts of 24 aratis or cubts of length. Satapatha Brahmana (XIII.4.4.5) speaks of 21 sacrificial stakes, all of 21 cubits long. Signs of poverty and prosperity are also known from sacrificial stakes.
If yupa is bent at the top and bent outwards in the middle it leads to poverty but the sacrificial stake bent at the top and bent inwards in the middle leads to prosperity (Satapatha Brahmana XI 7.3.3). The sacrificial stake of Vajapeya is 17 cubits long and looks like a thunderbolt. Sometimes a mountain is compared with yupa (Atharva Veda XIII.1.47).
Different views on the length of yupa are noticed in Srauta Sutras like Apastamba (VII.2.11-17, Katyayana, VI.I.24-26). It is also stated that yupa may be of any length, one to thirty three cubits and may be of four-cornered or eight cornered (astasri) or sixteen cornered and contracted in the middle with a wooden head-pieces like a mortar called causal.
Source -
Asian Mythologies (1993) by Wendy Doniger and Yves Bonnefoy Published by University of Chicago Press
Encyclopedia of Hinduism Volume XI page 643-44 - Published by IHRF - Rupa