As far as I know, it's a bunch of specifications for Java, of which there are probably a few reference implementations and for the popular specs several competing implementations (Tomcat, Jetty, Glassfish/Payara, Hibernate, EclipseLink, ...)

The specification part means that in theory one can take code written for one container runtime and move it to another that offers better $nonFunctionalRequirement (or licensing story) without code changes