file:/// is a URI (Uniform Resource Identifier) that just makes a distinction from the http://, which is the default URI that we are all too familiar with.
Although in any setting it does indicate an absolute path name pointing to the root directory, in the context of Android it is a convention to inform the Android run-time that "here, the directory www has a file called index.html placed in the assets folder at the root of the project."
This is how assets are loaded at runtime; for instance, by giving the file:/// URI, a WebView widget would know exactly where to load the embedded resource file.