From original project's page:

PPAPI-host NPAPI-plugin adapter.

As you know, Adobe have suspended further development of Flash player plugin for GNU/Linux. Latest available as an NPAPI plugin version 11.2 will get security updates for five years (since its release on May 4th, 2012), but further development have been ceased. Fortunately or not, newer versions are still available for Linux as a part of Chrome browser, where Flash comes bundled in a form of PPAPI plugin. PPAPI or Pepper Plugin API is an interface promoted by Chromium/Chrome team for browser plugins. It's NPAPI-inspired yet significantly different API which have every conceivable function plugin may want. Two-dimensional graphics, OpenGL ES, font rendering, network access, audio, and so on. It's huge, there are 111 groups of functions, called interfaces which todays Chromium browser offers to plugins. Although specs are not final yet, and new interface versions are arising, with some older ones getting deleted; rate of change have significantly slowed down.

For various reasons Firefox developers are not interested now in implementing PPAPI in Firefox. However that does not mean it cannot be done.

The main goal of this project is to get PPAPI (Pepper) Flash player working in Firefox. This can be done in two ways. First one is to implement full PPAPI interface in Firefox itself. Other one is to implement a wrapper, some kind of adapter which will look like browser to PPAPI plugin and look like NPAPI plugin for browser.

First approach requires strong knowledge of Firefox internals, and moreover additional effort to get the code into mainstream. Maintaining a set of patches doesn't look like a good idea. Second approach allows to concentrate on two APIs only. Yes one of them is big, but still graspable. Second way will be used for the project. It will benefit other browsers too, not only Firefox.