The Unverified flag is used to signal a translation that you do not consider to be 100% correct.
You can mark this with the help of the Unverified button found at the end of the translation row and come back to it later in your process.
Contributors may filter just Unverified strings to see which translations require corrections.
There is an Auto-toggle Unverified option in project settings, that will automatically mark other language translations as Unverified in the case that the base language translation has been changed (within the editor, at import, over the API or using any other source).
Please note that if you mark the translation as reviewed, it will also become verified automatically.