The context path is the prefix of a URL path that is used to select the webapplication to which an incoming request is routed. Typically a URL in a java servlet server is of the format http://hostname.com/contextPath/servletPath/pathInfo, where each of the path elements may be zero or more / separated elements. If there is no context path, then the context is referred to as the root context.
How the context path is set depends on how the webapplication (or ContextHandler) is deployed:
If Jetty is run from code as an embedded server then setting the context path is a matter of calling the setContextPath method on the ContextHandler instance (or WebAppContext instance).
If the webapplication is run from the webapp deployer/provider then by default the name of the WAR file is used to form the context path. So if the file is called foobar.war, then the context path will be /foobar. There is a special case for webapps called root.war which are mapped to the root / context.
The context path may also be set for webapps by embedding a WEB-INF/jetty-web.xml file in the WAR. This is not a preferred method.