The terma system is the mechanism by which the Nyingma school receives new scripture, and it is unusual in that the tradition is entirely open about the fact that the texts appear later than the figure they are attributed to.
The claim is that Padmasambhava, foreseeing periods when the teaching would be suppressed or misunderstood, concealed texts, objects and instructions in rocks, lakes, statues, pillars and in the minds of his disciples, to be found when they would be useful. The finders are tertons, treasure revealers, understood to be rebirths of those disciples. Earth treasures are physical objects recovered from physical places; mind treasures arise in the terton's awareness without any object being dug up. The best-known terma in the West is the collection published in English as the Tibetan Book of the Dead.
Two things follow, and they pull in opposite directions.
The first is that this is a working system for producing authoritative new texts inside a tradition that formally closed its canon, and it produces a great deal of the Nyingma literature, including the biographies of Padmasambhava himself. Nyangral Nyima Ozer and Orgyen Lingpa, who wrote the standard lives, were both tertons, so the biography of the figure who authorises terma is itself terma. The circularity is visible from outside and does not trouble the tradition, which does not share the assumption that a text's value depends on the date of its composition.
The second is that this makes historical work on Padmasambhava difficult in a specific way. The sources are not forgeries in intent: nobody conceals that the Padma bka' thang was written in 1352. They are texts making a claim about transmission that a historian cannot evaluate on historical grounds.
Other Tibetan schools have contested terma, and the Nyingma have defended it, since at least the eleventh century.