Koh is Thai for 'island(s)'. 'Koh Phi Phi' refers to the group of Phi Phi islands.
'Koh Phi Phi Island' is actually meaningless but almost certainly refers to the main island, Koh Phi Phi Don, which is the largest Phi Phi island and the only one which is permanently inhabited.
'Koh Phi Phi Le' is a small island (but the second biggest in the group), which the tour operators would describe as 'idyllic'. It was used as the location for the film 'The Beach'. If you've seen it referred to as 'Koh Phi Phi Le Beach', it seems that a tour operator is deliberately using a reference to the film.
Chris