JWPlayer is an in-browser multimedia player made by Longtail Video. Islandora uses it to create viewers for objects using the Audio Solution Pack and Video Solution Pack, so that they can be streamed directly inside a user's browser instead of requiring them to download it to their own computers first.
The actual JWPlayer files can be downloaded from http://www.longtailvideo.com/jw-player/download/
The Islandora JWPlayer Drupal module required for Islandora integration can be found at the Release Notes and Downloads page.
The process of installing JWPlayer is broken up into the following steps:
Check the Audio Solution Pack and Video Solution Pack pages' configuration sections for more information on how to integrate JWPlayer into Islandora.