"ROM" is literally Read Only Memory. In fact the phone doesn't really contain any of this.
However, in Android people call the system software a "ROM", because it lives in a part of the phone storage which the user cannot normally write to (unless they root the phone).
If you've read this in a spec sheet, it refers to the amount of the phone's storage which has been allocated to the system software.
In any case, it's not space which you can download apps to.