הלל (halal) is a verb that means to be boastful or to praise.
The word Hallelujah consists of two parts. It ends with יה (Yah) which are abbreviated forms of the Tetragrammaton; the name of the the Most High: YHWH. It begins with an imperative verb form, which means that it is an authoritative command of the root הלל (halal).
So the next time you say or hear someone say HalleluYAH, know that it is a command being given to praise the Most High Yah and not just a verbal expression to be repeated without action following.