what is a writable disc?

Answer 1

One you can "write" on -- that is, one onto which you can copy computer, video, or audio files.

A re-writable disk is one that you can copy material onto, erase, and copy different material onto.

CD-R or DVD-R = you can "write" onto the disk once, but cannot erase and add additional material

CD-RW or DVD-RW = you can "write" onto the disk again and again.