A Legacy Contact on Facebook is a person you choose to carry your legacy.  This person will have access to certain settings on your Facebook page and can make changes once the account has been memorialized like:

• Change the person’s profile picture and cover photo
•  Write a pinned post on the timeline
•  Respond to new friend requests

Once an account is memorialized, content can’t be removed from it. To choose a legacy contact, open up your Facebook account and go to Settings > Security > Legacy Contact. Click Edit, type in your friend’s name, and click Add.