The word "hallelujah" mentioned in Psalms is the Hebrew word for requesting a congregation to join in praise. The best translation of hallelujah is "Praise Yah, you people", usually worded in English versions as "Praise ye the LORD" or "Praise the LORD".
In the Hebrew Bible "hallelujah" is actually a two-word phrase, not one word. The first part, hallelu, is the second person imperative masculine plural form of the Hebrew verb hallal. However, "hallelujah" means more than simply "praise YHWH", as the word hallel in Hebrew means a joyous praise, to boast in God, or to act madly or foolishly.
The second part, Yah, is a shortened form of the name of God YHWH, sometimes rendered in English as "Yahweh" or "Jehovah". In the Hebrew reads kol han'shamah t'hallel yah; the final word "yah" is translated as "the LORD", or "YHWH". It appears in the Hebrew Bible as הללו~יה and הללו יה. In the Hebrew says "הללו יה hallelu yah". It then says "hallelu eth-YHWH" as if using "yah" and "YHWH" interchangeably. The word "Yah" appears by itself as a divine name in poetry about 49 times in the Hebrew Bible (including hallelu yah), such as in "who rides upon the deserts by his name Yah" and "Yah is my strength and song". It also often appears at the end of Israelite theophoric names such as Isaiah "yeshayah(u), Yahweh is salvation" and Jeremiah "yirmeyah(u), Yahweh is exalted".
The word Hallelujah appears in Revelation 19 in Greek transliteration as "allelouia", the great song of praise to God for his triumphant reign.
Among many Christians, the expressions of Hallelujah and Praise the Lord are acceptable, spontaneous expressions of joy, thanksgiving and praise towards God, requiring no specific prompting or call or direction from those leading times of praise and singing.