Hebrews 11:1 says,

"Faith is the substance of things hoped for, the evidence of things unseen."

The underlying Hebrew concept of faith (emunah) means faithfulness. It includes belief, but also implies activity, an eventual response on our part (just as "Shema" doesn't mean "hear" only, but rather "hear and act accordingly").

The first part of this verse tells us that our belief, and the ability to act on it, is tangible - it almost has material substance. We can experience it, and therefore know we have it. Our faithfulness is an already manifest part of Spiritual Reality, here in the physical, on earth, in us, Now! The idea is similar to Ephesians 1:13-14, where the Holy Spirit is said to be like a down-payment or "earnest money." I can know that all the promises of God - the things hoped for - will be fulfilled, because I've already received a small but significant part of them, this very real thing called faith.

Looking at the 2nd half of the verse, in what way is faith evidence of the things we can't currently see? For instance, we can't see Yeshua, or heaven, or the angels. However, we know from Ephesians 2:8-9 that faith "is the gift of God, not of works, lest anyone should boast." In other words, if I have faith, it's only because it was given to me by God. It's not something I'm capable of making on my own. Hebrews 11:1 is telling us that the gift is evidence of the Unseen Giver. One of the ways we can know that God is real is by His having given us faith in Him, something we could not manufacture on our own.