First of all, this is a false positive. Nikto reports this issue "Server leaks inodes via ETags" if there is a dash in the ETag header, which is by itself not an indication of anything.
An inode is a data structure used by the Linux file system. Every file and directory has an inode which stores its name, size and other data. Every inode has a number which uniquely identifies it. Apache HTTPD used this in the ETag for a while. The ETag is an identifier which should uniquely identify a file on the webserver, and the inode number is a number which uniquely identifies a file on the filesystem, so it seemed to make sense to use one for the other.
The inode number is technical information form the file system. If exposed, it cannot be used to exploit anything useful. Furthermore, it is only exposed on files that you can already download. It doesn't seem to a real security risk to me if a server exposes inodes.