The String translation module, part of the Internationalization (i18n) package, provides support for other modules to translate user-defined strings. It implements the
i18n_strings function, which in Drupal 6 was commonly known as the
tt function. The
i18n_strings function is an API function that lets other modules (such as the Taxonomy translation and Menu translation submodules) manage translations of user-entered strings. Depending on which modules you have enabled that use this feature, different text groups will be available for translation.
In contrast to Drupal's standard localization system, which always translates from English, strings will be translated from the source language. By default the source language is the site's default language, so changing the default language could break these translations. You can set which language is used as the source language via Administration > Configuration > Regional and language > Multilingual settings > Strings.