A stealth activity is an activity that is hidden from students but accessible if you have a link to it. Watch this video explaining stealth activities (duration 2.44 mins):
Note that the layout in this video is quite different to FLO, but the principles are the same.
Changing stealth settings from the front page of a topic
Turn editing on in your topic
For the activity/resource you want to stealth, in the Edit drop-down menu select Hide
The activity/resource will now show as Hidden from students
For the same activity/resource, return to the Edit drop-down menu. This time, select Make available
The activity/resource will now show as Available but not shown on topic page. When a student visits the page, they will not see the activity/resource but if you provide a link to it, they will be able to access it
Changing stealth settings from an edit settings page
Alternatively, if you are editing the settings of an activity or resource:
Open the Common module settings section
Go to the Availability menu and select Make available but not shown on topic page.
The activity/resource will now show as Available but not shown on topic page. When a student visits the page, they will not see the activity/resource but if you provide a link to it, they will be able to access it