The legend at the bottom says it means "no information".
So I have a user whose assistant manages her calendar.
When her assistant makes a change or adds anything to her calendar it doesnt show up on hers unless she closes outlook and reopens it.
Microsoft Exchange Server 2007 will reach end of support on April 11, 2017. For more information, see Resources to help you upgrade your Office 2007 servers and clients.
The Availability service for Microsoft Exchange Server 2007 provides calendar information for your users.
I had her give me permissions to her calendar and when I had her assistant make a new meeting request it showed on mine and her assistants however never showed on the primary persons calendar unless we closed outlook and reopened it.