This feature isn't available everywhere at the moment. You can only lock your profile in certain countries.
If you see 
on someone's profile, that person has chosen to lock their profile and show a limited view of their profile content to people that they are not friends with on Facebook.
on someone's profile, that person has chosen to lock their profile and show a limited view of their profile content to people that they are not friends with on Facebook.When someone locks their profile, only their friends will see the following:
- Photos and posts on their profile.
- Their full-size profile picture or cover photo.
- Their stories.
- New posts and photos.
In addition:
- Any posts that they've shared in the past to Public will change to Friends.
- Profile review and tag review will be turned on.
- Only a portion of their About info will be visible to everyone on their profile.
Other ways to manage your privacy:
If locking your profile isn't available to you, you can control your privacy in other ways through your privacy settings:
- Use Privacy Checkup.
- Choose who you share posts, photos and other information with.
- Edit basic info and choose who can see it.
- Change your story privacy settings.
- Turn on profile review.
- Turn on tag review
- Turn on profile picture guard.
- Control who can friend and follow you.
- Control who can look you up using your email address or mobile phone number.