An override is a file in the Joomla template directory that is loaded instead of the normal core Joomla file.
An override is a file in Joomla specified overrides directories, that is loaded instead of the normal core Joomla file. This is done in order to change the layout, or functions of certain parts of Joomla, without changing the core files, because these are updated and overwritten whenever Joomla is updated.
Overrides can be done on Components, Modules, Languages etc. If extension uses the MVC (model-view-controller) layout then you can use template overriding.
Layout overrides only work within the active template and are located under the /html/ directory in the template. The layout overrides must be placed in particular folder structure within the /html/ folder in order to work:
/templates /YOURTEMPLATE /html /com_content (this directory matches the component/module directory name) /articles (this directory matches the view directory name) default.php (this file matches the layout file name) form.php
In Joomla 3.x, overrides can be created automatically from the template manager. In earlier versions, the file to be overwritten has to be copied manually to the corresponding folder within your template.