CD-ROM stands for Compact Disc - Read Only Memory. What does this mean? Well, compact disc is straightforward - its a disc, and at the time when it came out (during the mid 1980s), it was physically very small, but with a large capacity. Hence, it is quite literally a compact disc.
As for read-only-memory, that means that when you put the disc in a computer, all you can do is access the data that is already contained on it, whether that's music, documents or images. The computer can only 'read' it, not alter it or delete it.
This differs from CD-R, which stands for 'Compact-Disc Recordable. This means that when you put it in your computer, you can 'record' data onto it - you can put your files and documents onto it so that you can physically transport them.
There's also CD-R/W which stands for Compact Disc Re-writable, which means you can put data onto it multiple times. You can put something on it, transfer it somewhere, then delete it and put new information on afterwards.
Hope this was helpful,
Josh