Multithreading refers to having multiple threads in a single process.
Multiprocessing means running multiple processes on a machine.
In multithreading the advantage is that the memory is shared and multiple threads can use the same area of memory requiring it to load just once given synchronization issues are handled properly. Processes are isolated from each other and cannot directly access memory allocated to each other.
Due to isolation processes dont interfere with each other. e.g. if one process crashes other processes are unaffected. In case of multithreading all threads within the process stop executing when a process crashes. If one thread misbehaves and corrupts the shared area of memory, other threads would be affected too as they too are dependent on the same shared memory area.