As more processes and data get added to memory (RAM), it is possible to run out of free space within memory. In the earlier days of computing this would have meant having to close programs in order to open new ones.
To solve this issue we created the concept of Virtual Memory (sometimes referred to as Swap). When a computer is running low on space in memory, it will move "pages" associated with some of the processes to a special reserved space in secondary storage. The CPU cannot directly access the moved pages in the reserved virtual memory on the secondary storage, so if that process is needed again it must be swapped back into memory in order to be used.
There is a loss of computing performance when using virtual memory due especially when used with a hard disk drive, however on newer SSDs the process of swapping processes between memory and the SSD should not result in a noticable drop in performance.
If a computer is useing virtual memory frequently it is recomeneded to upgrade the amount of memory installed in the computer if it is possible to do so.