"read only" is a common phrase in computers that means
you can only read it, not write it.
Read only can apply to these things:
- A read only memory (ROM) is (usually) a memory chip that has its contents hardwired at manufacture. These are usually cheaper than normal RAM to manufacture, and more importantly, the contents will be available at boot, and it doesn't require power to retain its contents.
Of course, these can be (one time) programmable and eraseable, but that's another story.
- In unix like operating systems, a filesystem may be mounted read only (usually with the -r option or alternately -o ro. This might be done to protect it from writes while copying its data, or perhaps because it is a cd-rom that you can't write to, or perhaps it is dangerous to write to the filesystem, either because it is damaged already, or because your operating system is not fully aware of the semantics of writing to a foreign filesystem.
- Your editor may tell you that you are editing a file read only because you don't have write access to the file, maybe because you don't own it.
- Many types of magnetic media can be made read only by adjusting their write protect tab. On 5 1/4" floppy disks, covered meant protected; on VCR tapes and cassette tapes, busted tab is protected; on 3 1/2 diskettes and DAT cassettes, slid open (or busted) tab means protected; quarter inch tapes (qic) are write protected by rotating it to the "lock" position.
In summary, things may be read only for safety, or due to hardware capabilities.