A general word for “person”, any individual independent of their gender and species, since the term was “freely applied to other Incarnates, such as Men or Dwarves, when the Eldar became acquainted with them” (WJ/372). In unstressed form quen, it was sometimes used as a pronoun “one, somebody”, and was also used as the second element in compounds such as ilquen “everyone” and roquen “horseman” (WJ/363, 372).
Most likely the Elves had a bias towards themselves as the main category of persons, since they used the term Quendi “(lit.) Speakers” to refer the Elves as species, and quén seems to have originally have been a variant of that term, derived from the same root √KWEN “speak”. This word was primarily discussed in the Quendi and Eldar essay from 1959-60, which may be where the term was introduced, but it appears in other late notes as well (PE19/93).
quén (quen-, as in pl. queni; as final element in compounds -quen) noun "one, (some)body, person, individual, man or woman", pl. queni = "persons", "(some) people", "they" with the most general meaning (as in "they [= people in general] say that..."). The element is combined with noun and adjective stems in old compounds to denote habitual occupations or functions, or to describe those having some notable (permanent) quality; examples include roquen, ciryaquen, arquen, q.v. Also in aiquen "whoever", ilquen "everybody" (WJ:361 cf. 360, 372).