We reported in June that Google was working on a similar feature to Microsoft’s Personal Vault on OneDrive for Android called Safe Folders.
Safe Folder is a PIN-protected folder on your handset which can only be accessed with the password, and which is invisible to other apps on your handset.
The data is encrypted with a 4 digit pin and Google promises that it is completely inaccessible without it, meaning if you forget your PIN the data is unrecoverable. The data is also lost if you delete and re-install the Files app.
XDA-Developers report that the Safe Folder feature is currently rolling out for users with Files by Google version 1.0.323, according to AndroidPolice. You can download this version from APKMirror or from the Google Play Store.
It is notable that a 4 digit PIN does not offer any significant protection, meaning Safe Folders may be enough to hide files from your wife but not the FBI. The data is also not synced to the cloud, so if you lose your phone you lose your data.