Old age and death are realities every human being faces. Krishna says: those who look at these realities honestly and, instead of running from them, turn toward Me and strive — they come to know everything. Brahman, the Self, the entire workings of karma — all of it becomes clear.
There is a practical directness here. Krishna does not ask people to ignore the fact of aging and dying. He says: use that awareness as fuel. Let it turn you toward the eternal. Like a lamp that illuminates everything in its reach, the shelter of the divine brings clarity about Brahman, about one's own inner nature, and about the full scope of action and consequence.
This shloka follows naturally from 7.28. The dvandva-free devotee of firm resolve — what does such a person come to know? Three things: Brahman, adhyatma, karma. The answer sets up the final shloka of the chapter.