This allows you to use Javascript in map/reduce views, as opposed to native Java code (the default behavior for Couchbase-Lite).
To "activate" this, you will need to include this library and it's dependencies, and add the following code when you initialize Couchbase-Lite:
CBLView.setCompiler(new CBLJavaScriptViewCompiler());
See couchbase-lite-android-liteserv for an example where this is used.