inodes are data structures that contain information about files in Unix file systems. Each file has an inode and is identified by an inode number (i-number) in the file system where it resides. inodes provide important information on files such as user and group ownership, access mode (read, write, execute permissions) and type.
inodes are created when a file system is created. There are a set number of inodes, which indicates the maximum number of files the system can hold.
A file's inode number can be found using the ls -i command, while the ls -l command will retrieve inode information.
[above from webopedia.com]
Essentially, when your sysadmin created the file system he didn't allocate enough inodes.