The word "Alleluia" or "Hallelujah" (from Hebrew הללו יה), which literally means "Praise ye Yah" or "Praise Jah, you people", is used in different ways in Christian liturgies. "Praise Jah" is a short form of "Praise Yahweh", or of "praise ye Jehovah".
In the spelling "Alleluia", the term is used also to refer to a liturgical chant in which that word is combined with verses of Scripture, usually from the Psalms. This chant is commonly used before the proclamation of the Gospel.
Read more about Alleluia: History