This entry refers to strings which are written in the code of your plugin or theme. For strings entered in the theme or plugins options, you must use the Translations table.
Themes and plugins translations are handled in .po files ( human readable ) and .mo files ( machine readable ).
If your theme is correctly internationalized ( translation ready ), you can use Poedit or a plugin such as Loco translate to do the translation.
Plugins and themes available on wordpress.org can be translated online with Translating WordPress.
Ressources for developpers:
Child themes internationalization
How to Internationalize Your Plugin