The Sanskrit word Rudra has a number of meanings. It matters to the International Nath Order as an ancient god-form related to the later name Shiva. Rudra is spoken of as a controller of nature and as a remover of suffering.
The word can mean the Howler, or ruddy. Mahendranath also names Shiva as Kalagni-Rudra—Rudra of the Fire of Time. In the INO this is a form of the Cosmic Spirit, not a second god and not a creed. Bhairava is the fierce office of the Lord of Magick. Rudra is the older name. Rudraksha is the seed said to have sprung from Rudra’s tears; that story is Kapilnath’s. This page names the god-form. It does not teach a mantra.