“So, from primordial time, all that is ultimately consumed within the exalted dimensions, types of pristine awareness, qualities, and enlightened activities abides within the essence of the tathgatas. When someone becomes a Buddha, these are not then newly acquired but are merely separated from the defilements that have eclipsed them.”
[From The Essence of Zhentong by Jetsun Taranatha]
In other words, according to this teaching the innate quality of your mind is ultimate clarity, or Awakeness. However, for most of us this Awakeness is obstructed by habitual patterns, conditioning, and various forms of obscuration. This situation has been likened to a mirror covered in dust and grime. The mirror is there beneath the dust. Wipe the dust away, and the mirror will shine forth.
According to this teaching, buddha-nature is not something you presently lack, and can later acquire. Buddha-nature is something you already have, it is the very essence of your mind. The difference between ordinary beings and fully enlightened buddhas is that the latter have removed the obstructions that prevent this Awakeness from shining forth. This is a task of Dharma practice, to remove the obstructions and obscurations. There are innumerable methods available for doing this, applicable to beings of different capacities and dispositions.
OM AH HUM