This means there is not enough memory (RAM) for your computer to keep all of the programs that you are running stored there. Blocks of memory are freed up for programs that need them, and background applications [those not in use, for example, minimized on your desktop] are temporarily stored on the hard disk until those portions of programs or data are called for by the system [in other words, you make them active by clicking on them]. Then the computer reloads the stuff from hard disk into RAM to run the program.
If you are not having performance problems (like long waits for response], don't worry about it. But, if you do a lot of switching between programs, that process could be sped up by adding RAM - the computer would not have to shuffle stuff back and forth constantly from the hard disk to RAM where the CPU can work on it when you switch programs.
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