IMAP Folder Subscription is a standardised method to allow
different clients connecting to the same user mail folders to
control which folders are made available.
You can see it working using the following steps:
1. Create a test folder in your horde webmail.
2. Select checkbox next to it and then select
    "Unsubscribe from folders" from the "Choose Action" 
     list.
3. Now this folder will not be visible if you have 
    selected "Hide Unsubscribed" option. Alternatively,
    You can open squirrelmail for the same mailbox
    and this folder will not be visible there.
4. To get this folder back, just select checkbox 
    next to the unsubscribed folder and select 
    "Subscribe to folder" and now the folder will be 
    visible in both email clients (Make sure that you 
    have selected the "Show Unsubscribed" option
    to view the unsubscribed folder ).
This facility, i suppose, is aimed at usage of shared/public 
folders made available by IMAP Servers and not for personal
folders.
Hope it helps.