Yesh (יֵשׁ) is a Hebrew particle of existence, equivalent to "there is" or "there are." It appears roughly 138 times in the OT and is the positive counterpart to ayin (H369, "there is not"). The word asserts the existence, presence, or availability of something.
It functions differently from the Hebrew verb "to be" (hayah, H1961). While hayah describes states and events, yesh affirms simple existence: "there is a God" (yesh Elohim) or "there is hope" (yesh tiqvah).
In theological usage, yesh makes bold affirmations about God's reality and provision. Proverbs 8:21 — "granting an inheritance to those who love me, and filling their treasuries" — is literally "there is substance [yesh] for those who love me." The word stakes out the reality of divine blessing against emptiness.
The contrast yesh/ayin structures the Bible's great either/or: either the LORD exists and acts (yesh) or he does not (ayin — the fool's claim in Psalm 14:1). "Is the LORD among us or not?" (Exod. 17:7) is literally yesh/ayin. Every miracle answers the doubt with a resounding yesh.