What is an MP3 CD?

First thing you must know is that there are two different kinds of CDs: Audio CDs and Data CDs. And Audio CD is a CD that contains recorded music only, such the ones you can buy in a music shop. Data CD is a CD that contains files readable by a computer, such like a videogame CD you buy in a computer shop.

The difference is not physical (they both are physically made the same way) but it stays in the way information is recorded on them.

An MP3 CD is a DATA CD which contains music stored in one or more (usually many) MP3 files.

Thus you need devices enabled to read MP3 CDs, and taht's why some old stereo with audio cd reader can't read MP3 CDs.

So if you are going to create a MP3 CD, you are just creating a DATA CD with some MP3 files on.

If you are wondering if creating a Audio Cd from MP3 tracks allows you to store more tracks than the original in the same CD (like 100 tracks instead than 12), the answer is no, in fact on Audio CDs music is stored as uncompressed PCM format (basically the same format of WAV files), so your CD burning software have to decompress MP3 files in order to store them to the Audio CD.

Now, if you are going to create an MP3 CD, you need to get some MP3 files. The easiest way is to extract tracks from your audio CD collection and encode them to MP3 with FreeRIP. Once you have a set of MP3 files, use your favourite CD burning software to burn them to a data CD.