{jrCore_lang} shows a language string: a piece of text that can be translated. Skins and modules use it for all their text, so a site can change or translate it on the Language tab.
{jrCore_lang module="jrEvent" id="33" default="Updated an Event"}
{jrCore_lang skin=$_conf.jrCore_active_skin id="12" default="Newest Members"}The first shows string 33 of the Event module: $lang[33] in modules/jrEvent/lang/en-US.php.
When a module or skin is installed, its strings are copied into the database. Changing the lang file afterwards doesn't change the site: change the string on the module's or skin's Language tab instead.
New strings you add to a lang file are added at the next Integrity Check.