When a file is created, it's given a link count of one. As you add and remove links to and from the file, this link count is incremented and decremented.
The disk space occupied by the file data isn't freed and marked as unused in the bitmap until its link count goes to zero and all programs using the file have closed it. This allows an open file to remain in use, even though it has been completely unlinked. This behavior is part of that stipulated by POSIX and common Unix practice.