ROM shorts for Read-only Memory, and it is a kind of memory that stores data long-term. This memory is non-volatile, meaning it keeps data even when the devices is powered off. This memory stores user content, as well as a copy of all software so it can be loaded into RAM when needed. ROM is better known as flash memory. "Read only" is actually a misnomer that has carried over from older devices that had an OS and applications that where generally fixed and therefore stored in truly "read-only" memory. Modern devices use storage memory that easily and frequently changed, but the term has remained.
16GB of ROM is always used for some low end or midrange smartphones that priced between $60 - $100.
Common ROM examples: 2GB, 8GB, 16GB, 32GB, 64GB, 128GB.