Brahma Granthi in spiritual terms means the knot of ignorance. Granthi in Sanskrit means is a gland or a knot. Brahma is the God of creation. In Katha Upanishad (II.2.15) and Mundaka Upanishad (II.2.8), it is called hridaya granthi (knot in the heart), and it is said that, “realization of the Supreme Reality cuts the knot of ignorance in the heart.” The word heart here means buddhi (intellect), which is the abode of ignorance as well as knowledge.
In Hatha Yoga three knots of ignorance are recognized, namely Brahma Granthi (the knot of Brahma), Vishnu Granthi (the knot of Vishnu) and Rudra Granthi (the knot of Shiva).
They are situated, respectively, in the three energy centers (called lotuses) at the heart (called Anahata chakra), thorat (called Vishuddhi Chakra) and eyebrows (called ajna chakra).
These energy centers and knots are closed in all ignorant beings. They are opened by a rigorous practice of breath control, i.e, pranayama.
As described in Hatha Yoga Pradipika (IV 70-76), an adept parts hearing subtle sounds, i.e, nada when after the arousal of the dormant kundalini power and its upward ascent, the heart lotus and Brahma granthi are opened.
Subsequently, with the continued practice of hearing the sound, the other two knots are also opened. Different sounds are heard which are gross in the beginning but they become finer and subtler with progress in practice. This is called nadanusandhana, which is the fourth and final part of the fourfold practice of hatha yoga.
Ultimately, the sounds become imperceptible, and the mind becomes absolutely silent and absorbed in the state of Samadhi, which brings emancipation.
Opening of the granthi thus opens the door to the highest spiritual goal.