From the Puranas, one can infer that the meaning refers to either the spiritual aura of a person, facial expressiveness that exudes confidence, vigor or valor, etc. Real Gurus are said be endowed with the ability to judge a wannabe-disciple's worthiness by just observing his/her Tejas. In absolute spiritual terms, what exactly does Tejas mean? Which state or evolution of which Kosha among the five koshas - Annamaya, Pranamaya, Manomaya and Anadhamaya does Tejas indicate?
Tejas means heat. When prana becomes active, it generates heat - tejas. Tejas is one of the fine elements (Akasha, Vayu, Tejas, etc.). There is both a cosmic as well as a personal aspect to the meaning. When the cosmic Akasha is repeatedly given blows by Prana, Tejas (heat) arises (Swami Vivekananda). From a raja yoga aspect, repeated blows to the Kundalini awakens the Kundalini. The movement of the Kundalini gives rise to Tejas - heat.
Tejas can thus mean spiritual heat, or power generated by the Kundalini. When it is said that a real teacher can observe an aspiring student's tejas, it means that the preceptor can perceive how much effort needs to be done on the student's part to awaken the Kundalini. With tejas, the Kundalini can arise and open up spiritual vistas to the student.