If you mean the naked
- at the end of the
tar command, that's common on many commands that want to use a file.
It allows you to specify standard input or output rather than an actual file name.
That's the case for your first and third example. For example, the
cdrecord command is taking standard input (the ISO image stream produced by
mkisofs) and writing it directly to
cd command, every time you change directory, it stores the directory you came from. If you do
cd with the special
- "directory name", it uses that remembered directory instead of a real one. You can easily switch between two directories quite quickly by using that.
Other commands may treat
- as a different special value.