In the June Pixel Feature Drop, Google introduced “Locked Folder” within the Google Photos app to Pixel devices. As initially reported by Android Police, the feature is now rolling out non-Google Pixel devices.
Locked Folder is a secure on-device folder where you can store private (yeah, uh huh) pictures and videos and they won’t be uploaded to Google Photos. From here, you can only view whatever you stored, and you won’t be allowed to share, screenshot, or screen record any of the content unless it’s removed back out of the Locked Folder.
Source: Android Police