What is RAT?

Basically, you can think of it as a tool that checks, whether you're source files are carrying proper license headers. Almost any non-trivial organization has a policy for license headers. In the case of the Apache Software Foundation (ASF), this is the ASF Source Header and Copyright Notice Policy.

Where do I find RAT?

Rat can be found http://incubator.apache.org/rat/. In other words, it is an incubating Apache project: Has been preliminarily accepted by the ASF, but not finally.

Why should I use the Maven plugin to run RAT?

Because it makes running RAT trivial. Basically, you just add the plugin to the build/plugins section of your POM and run mvn rat:check. Using RAT cannot be simpler.

The apache-rat-plugin creates a report for every subproject. I want a single report for the whole project, which includes the subprojects.

Set the configuration parameter "excludeSubProjects" to false.