The PacletInfo.m is a descriptor file for Mathematica applications. In the current version of Mathematica it is only used by the documentation system. This means that if you provide documentation for your application you must provide a PacletInfo.m, and Mathematica will locate your documentation based on its settings. Other contents of the application, such as Mathematica code, are found by installing the application on the Mathematica $Path.
A sample PacletInfo.m is shown in the following.
The PacletInfo.m contains a set of rules that describe the application and what it contains. You can see details in the contents section.
The paclet system lets you install more than one application of the same name, so long as the PacletInfo.m have different version numbers. When selecting which one to use, it looks at all that are suitable for that version of Mathematica and picks the one with the highest version number.
The Wolfram Workbench provides a special multi-page editor for the PacletInfo.m that helps you to work with it. When you open the file you see an overview page, with tabs to navigate to the other pages. An example is shown below.
If you change anything in these fields, you will need to save the editor.
You can visit the other pages of the editor by clicking on the respective tabs.
The Extensions tab lists all the extensions in the PacletInfo.m. Typically, this is just going to list the Documentation extension. A sample follows.
The details of this tab are described in the section on the documentation extension.
The Documentation tab shows the documentation you have written for your application. An example is shown below.
From this page you add or remove documentation pages. There are also special tools for creating symbol pages and an overview page.
The PacletInfo.m tab shows the raw source of the file. You can edit the contents of the file directly, but typically this is not necessary since the other tabs provide a way to set it up.