Hello.
ROM: Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM can only be modified slowly, with difficulty, or not at all, so it is mainly used to store firmware (software that is closely tied to specific hardware. Read more https://en.wikipedia.org/wiki/Read-only_memory.
On Board Memory: Refers to memory chips (for storing data and files) on the main board of your android device/PC (Not expandable).