When you add activities to a timeline you can optionally mark them as confidential, this means that these activities can only be viewed, updated or deleted by users who have been provided access to Confidential Activities.
Allow access to Confidential Activities
To allow standard or viewer users to work with confidential activities:
Navigate to Settings.
Click on Users on the Existing Users tab.
Find the user you want to allow access to confidential activities.
Open their record.
Set Allow access to Confidential Activities to Yes.
Users with access to confidential activities will see a switch when they add or update an activity that allows them to mark the activity as confidential.
Users with access to confidential activities will also see a switch when working with activities lists which allows them to include confidential activities.