Why doesn't my CD / DVD work on my iMac ?

If these are CD-Rs, did you burn them on a really old Mac?

Once upon a time, Macs used the HFS (no "+") filesystem when burning data CDs. I believe that you could choose whether to write a Mac-ony CD, or a hybrid CD that would be readable on Windows PCs as well. These hybrid CDs had a second filesystem that Windows could recognize without extra drivers.

Starting in Catalina, Apple dropped all support for HFS (no "+"). (Support for HFS+, a.k.a. Mac OS Extended, is still present in Sonoma.). Modern Macs that encounter CD-Rs with HFS on them (whether they are hybrid CD-Rs or not) will reportedly choke and refuse to read them.

I think Apple dropped the ball here – but until they fix things (if they ever do), your best bet for recovering data from such CDs might lie with old Macs (that can run pre-Catalina versions of macOS) or with Windows PCs.

I remember that Kodak also had a couple of special CD formats for Picture CDs and Photo CDs.

If any of those required you to run Kodak-supplied software (on the CD) to access the pictures, you might want to Google and see what it takes to read those on modern machines (if you can).