"Substance" is a mechanical rendering of the underlying Greek word, which is hypostasis. "Understanding" would have been a similarly mechanical rendering, and similarly uninformative. Hypostasis is a word that refers to a shared substance or reality; it has been adopted by Christian theologians to refer to each/any member of the Trinity. Christ, the Logos, or Aramaic Memra are said to be (or not be, in some case) a hypostasis, an appearance of divinity sharing divinity's substance.
So "incarnation" is a word that could translate hypostasis, and shed more light. Faith is the incarnation of things hoped for, the "coming to light" of deeds not seen. "In faith" (not by faith) the elders have "been witnessed" (not obtained a good report).
This is ordinance language. In faith, Hebrews has seen incarnated and brought to light that which is ordinarily behind the veil, and also the elders. In this same context, "we" understand God's creation of the earth.