Separately, hallelujah means praise the Lord, while hollaback means reply. So, combined, it means praise the lord for answering my prayers. Usually used when the Lord sends a fine piece of booty across your field of vision.
An expression used when departing. Can be used interchangeably with the words "i'll holla 'atcha", "later", or "bye". Was originally brought into existance by some douche bag from the "The White Rapper Show" on VH1.