The PacletInfo.m is a descriptor file for Wolfram Language applications. In the current version of the Wolfram Engine 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 the Wolfram System will locate your documentation based on its settings. Other contents of the application, such as Wolfram Language code, are found by installing the application on the Wolfram Language $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 the Wolfram Engine 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.