Katha is the verbal explanation/discourse of Gurbani and our great history. Katha has been an integral part of Sikh practice since the revealed inception of Sikhismby Sri Guru Nanak Dev Jee.
Sri Guru Nanak Dev Jee and great Sikhs of his era performed Katha and it was and is still through Katha that many people gain knowledge about Sikhism and become enlightened. Katha is both a spiritual and historical discourse, it endows the listener with spiritual and worldly knowledge, and helps Sikhs along the path of Sikhi.
Sri Guru Amar Das Jee enlightens us about Gurbani Katha in the following manner,
Through the Guru’s Teachings, they intuitively taste the sublime, exquisite and ambrosial sermon of the Lord. (SGGS Ang 87)
urbani Katha is also a form of Amrit, as the teachings verbally expressed in Katha make us reach this sublime stage of immortality. In the same manner that Naam/Gurbani is Amrit the discourse of this Naam/Gurbani is also Amrit as Sri Guru Amar Das Jee taches us in the above quotation.
The filth of our bad actions is removed by listening and practising the teachings of Katha, as Sri Guru Arjan Dev Jee explains to us.