A virtual memory system is a computer system that allows a computer to use more memory than it has physically installed. It does this by temporarily transferring data from RAM to a hard disk, allowing a computer to use more memory than it has physically installed. This is useful when a program needs more memory than is available in the system.
For example, a system with 4GB of RAM may be able to run a program that requires 6GB of RAM. The virtual memory system will transfer 2GB of data from RAM to a hard disk, thus allowing the program to run with the extra memory it needs.