This plugin bundles JAXB libraries, and is a so-called library plugin.
JAXB libraries were removed from the JDK in Java 9. For a plugin using the JAXB API and built against older LTS lines, it is designed to be automatically added as a dependency when running Jenkins on a Java 11+ runtime, so that the plugin does not fail to link. A plugin using the JAXB API built against the 2.164.x or newer LTS line should declare an explicit dependency on this library plugin.
See the CHANGELOG.
See its plugins' website page on https://plugins.jenkins.io/jaxb