not enough memory sign.

I have a Dimension L933r. I need help to figure out what this warning means and what I can do  about it,(not enough memory), & short of getting a new computer. I'm try to lean computer terminology but, I have a long way to go, having said that, If any of you smart people out there can help with this problem I hope you can walk me slowly thru it using laymen terms. At this point I don't if i need to look at the "C" drive or hard drive.

                                                                          DON