This module adds support for other modules to translate user defined strings. Depending on which modules you have enabled that use this feature you may see different text groups to translate.
This works differently to Drupal standard localization system: The strings will be translated from the source language, which defaults to the site default language (it may not be English), so changing the default language may cause all these translations to be broken.
Read more on the Internationalization handbook: Translating user defined strings.