In a multiprocessor computer system, there are multiple central processing unit (CPU) that share full access to a common random access memory (RAM).
In a multiprocessor system, a program running on any of the central processing units sees a normal virtual address space.
In a multiprocessor system, the only unusual property is that the central processing unit can write some value into a memory word and then read that word back and get some different value because another central processing unit has changed it.
Now, when organized correctly, then this property forms the basis of interprocessor communication in which one central processing unit writes the data into memory and another one reads that data out.
You will learn all about multiprocessors in operating system in detail, divided into following tutorials: