Cannot BackUp Twice to CD-RW - Why?

I'm a absolute Newbie to the Mac. Today, I almost lost something I've spent days working on so I thought I'd try to avoid that happening again. I have a box of brand new CD-RW. I opened one, loaded it, dragged the folder to the CD icon on the desktop, then "burned". All seemed good. Went back to work at the end of which thought well I'll backup again. No can do! The CD tells me I cannot do this because the "CD >>name<< Cannot Be Modified". In all innocence perhaps, I thought one could write many times to a CD with RW on it. Anyway, I'd be very grateful for advice & my apologies if I'm asking a really dumb question. In case it helps, mine is a MacBook Air & I'm using the MBA Superdrive

 

Well, thank you both. It all appears to be much more complicated than I thought. I haven't installed any special software for writing/burning or whatever to CD-RW. Stupidly, in retrospect, I suppose I thought that would be built into my MacBook Air. After dragging the folder to the CD-RW icon, I was only given the option to burn so far as I can recall. So there must be some primitive software on board. However, there was nothing said about multi-session or finalising or anything else. Goodness, surely when I've spent all this money on an Apple Mac machine I don't have to rush out to buy software so I can copy stuff to a CD and then update it periodically!? Anyway, thanks again for helping me.