The subject-matter is sanwe-latya (Quenya for "thought-opening", but better translated as "telepathy"). Pengolodh included it as an appendix to the Lhammas because of the implications of tengwesta ("grammar, language") on thought-transmission, i.e. with the development of language, telepathy became more difficult and all but fell out of use among the "incarnated" (those beings with hröa).
The essay was written in c. 1959-60 by J.R.R. Tolkien as a typescript of eight pages and was published in Vinyar Tengwar, Number 39, July 1998, edited by Carl F. Hostetter. From the same editor, the essay was later included as a chapter of The Nature of Middle-earth (2021).
A related manuscript, "Etymological Notes on the Ósanwe-kenta", was published in Vinyar Tengwar, Number 41, July 2000.