Can mean many things - what's the context? Is it something you've seen written on a drive specification?
Rather than have yet another drive, why don't you buy a DVD writer (which will also read DVDs) and then stick to the CD-RW drive - ditch the DVD read only drive.
The secondary IDE cable will then be free to take both drives, and you should set the new DVD writer to be the master on this cable - set the CD drive as the slave.
When you've finished, go into your BIOS and set the CD drive (the slave) as a boot device in the bootable drives list, so that your computer will boot from a CD if ever it's necessary.