If you want to prevent people you do not know to send you invitations to be friends, you can restrict this to friends (which is useless) and their friends.
This will prevent you ever receive invitations fake accounts created for spam only.

To limit these invitations, click the small gear in the upper right and then "Privacy".
Then click on "Who can send me friend requests ?" and select "Friends of Friends" in the list.

Now, if you want to add a person to your friends, it will send the invitation to accept it. Or temporarily change this option.