That is a critical privacy question to ask.
Short Answer: A non-friend can only see a small, static version of your profile and cover photo, your name, and a tiny part of your ‘About’ info. All your posts, photos, and stories are completely hidden.
This feature is designed to put a privacy wall around all your significant content formats, making them visible only to your confirmed friends.
When a stranger lands on your locked Facebook profile, they cannot click or enlarge your profile picture or cover photo; they just see the small thumbnails. More importantly, all your content formats are hidden. This includes every text post, image, or video you have ever shared, even if you had previously set that content to ‘Public’. The ‘Lock Profile’ function retroactively changes all your past public posts to ‘Friends’ only and ensures all future content is set the same way, giving you a comprehensive privacy shield.
Cheers,
Jeff