as said above it means that it's a private post locked only to you and another person (or multiple people if you want) so that you can discuss the line ooc. or if they've asked ic one, it's a private spam post that their character can use. basically in the end it's just a space that only the two of you can see. in case you don't know how to make one i wrote this up super quick:
1. friend the journal they're using 2. go here 3. click 'new' and make a name for the custom (not important what it is) 4. go through the list of usernames in the 'not in group' list (ie: it lists anyone you have added to your friends list) and find the username you're setting up the custom for, select it, and hit the » arrow buttons so the username is in the 'in group' section. 5. hit save
THEN (as if this wasn't longwinded enough) you make a new post and in the dropdown menu where you select either everyone (public), friends, or just me, you'll instead pick 'custom...' and if everything has been done properly you'll see the title of the custom group you've created. click the ticky box beside whichever one you've created and post the entry. it should be locked to the person you've created the custom for if all is well.