@XmlRootElement is a JaxB annotation. This is not a Spring class but is part of the standard Java libraries.
You often see it used in Spring however in conjunction with Spring's JaxB HTTP Message Converter. When you see an @RequestBody annotation on an object in a Spring controller, and the consumes attribute of the @RequestMapping annotation is "application/xml" (or pre spring 3.1 the Content-Type header is set to "application/xml") this will tell Spring that the payload of the request message is XML and needs to be unmarshalled to the object that was annotated with @RequestBody. In this case the object that was annotated with @RequestBody will typically be annotated itself with JAXb annotations to provide the meta data needed to marshal and unmarshal that object to/from XML, and @XmlRootElement is one of those annotations providing that metadata.