Something that is only popular because it is popular, not being superior to similar items in any way. Usually also ridiculously overpriced due to it being perceived as a "status symbol" by the mainstream.
The iPod is the prime example of this phenomenon and has therefore given rise to the term.
Kid 1: Yo dude check out my sweet new iPod it holds 4 billion songs, 5 years of video, cooks and does the dishes!
Kid 2: Pfff...thats ipop u should have saved your cash for some booze fool!