Each unit may have several alternate translations associated with it. Such translations may appear during file update, when the source string is slightly changed. In this case the old translation with it's (old) source is moved to alternate translations list, so that they are not lost.
When translating software, usually gettext tools are used to prepare translation files. When original text changes, gettext tools update translation files and mark entries with changed original text as fuzzy (or non-ready in other terminology). They store previous original text so that translators could see what changes exactly were made. Lokalize simplifies the life of the translator and highlights parts of the original text that were changed in the Alternate Translations view.