SquirrelMail uses the subscription facility on the IMAP server to determine which folders to display. You may have many folders, but do not need to see all of them in SquirrelMail. Subscribing allows you to set your preferences as to which folders you want to see in the folder list. Unsubscribing does the opposite, it hides the folders that you do not want to see.
Please note that unsubscribing does not delete your folders or prevent your access to them. You are simply telling SquirrelMail that you do not wish to see a particular folder or folders at present. You can subscribe (bring back to view) any unsubscribed mailbox at any time.
Click on the Folders option at the top of the SquirrelMail screen.
Scroll down until you see the Unsubscribe/Subscribe box:
Select the folder and press the subscribe button to add it to your subscribed folders list (the mailboxes you want displayed in your folders list).