Internally at a low level a UNIX filesystem on a disk has a table of file information indexed by number; historically that index number is called an "inum". Directories are basically tables that associate a filename with an inum. It's possible for multiple pathnames in a filesystem to resolve to the same inum, so the inum is really the filesystem-unique identifier for a file, not the pathname. (Since usually multiple filesystems are mounted, joining them into the global directory tree, the globally-unique internal identifier for a file is a combination of the device identifier for a filesystem combined with an inum in that filesystem.)
The inum indexes a record called an "inode" which actually contains all the file "metadata" (information about the file distinct from its contents).
Making another link to the same file is now referred to as a "hard link" to distinguish it from a newer mechanism called a "symbolic link". Hard links must be to files in the same filesystem since pathnames must ultimately resolve to the same inum, and a file must already exist in order to create the second link to it.