Describe MassTranslated here.
this tag just shows that the page you are reading has been mass-translated, that is, a great number of pages have been translated at once. You will notice some mistakes like broken links and weird remants of HTML tags, though.
seeing this tag without the companion tag ManuallyAdjusted doesn't mean the translation was perfect, it may also mean I was too lazy to put the second tag later.
The MediaWikiMigrationReportStatus give some notes about errors from translations.