By default, when updating a record, existing data in the field are visible and can be edited. This feature allows you to prevent users from seeing existing data (cell values will be hidden and the field always starts empty) but at the same time, new values are still allowed to be added.

This option is available for some field types. Linked records, long text, & attachments. To find it go to

Allowing employees to add new notes/comments to a record without being to able to edit the existing notes.