Abarim Publications' online Biblical Hebrew Dictionary
The curious little word יש (yesh), marks existence. It occurs as proper noun in Proverbs 8:21 (as the difficult to translate opposite of absence or nothing-there-ness), but everywhere else it "draws attention to the existence or presence of an object or a quality" (says HAW Theological Wordbook of the Old Testament). It often simply means 'there is' (Genesis 31:29, Judges 17:13, 1 Samuel 20:8) or 'there are' (Judges 18:14, Ecclesiastes 6:11).
Translations often utilize italics to indicate the presence of this word: "Surely, YHWH is in this place!" (Genesis 28:16). This word is often used by itself, in answer to a question that contains it: Q: Is the seer here? A: He is (1 Samuel 9:11-12). BDB Theological Dictionary adds that our word occurs often in questions where a doubt is implied (1 Kings 18:10, Jeremiah 5:1).
Note that the Hebrew language doesn't use the verb to be as English does. In fact, if the verb היה (haya), meaning 'to be' is used, it indicates defining behavior. When someone is a carpenter, he is busy being a carpenter; that is, he's not simply 'there' but he is acting out the behavior that marks a carpenter (see our article To Be Is To Do).
Our word יש (yesh), in contrast, shouldn't be regarded as a verb of action, but rather a static assertion that someone(s) or something(s) exists in a given context. That means that, for instance, 2 Chronicles 15:7 does not assert that there 'will be' a reward for Israel's hard work (as older translations suggest), but that 'there is' one (as per newer translations).
Since our particle competes with the verb היה (haya), which expresses defining action, and Hebrew's innate propensity to express mere existence only sparsely, our particle occurs less than 140 times in the entire Bible.