This plugin provides the capability to build and sign jars.
Instructions on how to use the Jar Plugin can be found here.
The plugin use Maven Archiver to handle jar content and manifest configuration.
You can have a look at the Maven Archiver Documentation to understand how to setup this.
You might also want to consult the Guide to Working with Manifests.
To provide you with better understanding of some usages of the Jar Plugin, you can take a look at the following examples: