Mintus
Said..
In computer operating systems that have their main memory divided into pages, paging (sometimes called Memory swapping) is a transfer of pages between main memory and an auxiliary store, such as hard disk drive. Paging is an important part of virtual memory implementation in most contemporary general-purpose operating systems, allowing them to easily use disk storage for data that does not fit into physical RAM. Paging is usually implemented as a task built into the kernel of the operating system.
When the processes running on your server attempt to allocate more memory than your system has available, the kernel begins to swap memory pages to and from the disk.
This is done in order to free up sufficient physical memory to meet the RAM allocation requirements of the requestor.