Import notifications track your master files for changes that may not require changes in translated versions. These changes are e.g. small typo fixes. By default, these strings are marked as untranslated and translator then verifies them (and usually just copy the translation from Translation Memory). Import notification tool lets you "Keep the existing translation" so the previous translation is kept and copied back to this new string in all languages.
When we detect a change in a string, it's imported as a new string and will be empty by default. If the change is very small (e.g you fixed a small typo that necessarily doesn't change the meaning of the string), you get an import notification that let you either "Keep the existing translation" or "Mark string for retranslation". If you choose the keep the existing translation, old translations will be copied to this new string.
If you wish to dismiss these notifications, you can just ignore them by selecting them all and choosing "mark them all for retranslation". In this case translators will then review new strings and either retranslate them or copy the existing translation from translation memory.