It probably means that an application on the computer is grabbing all of the memory.
Press Ctrl-Alt-Delete (that's all three keys simultaneously) and click on the 'Processes' tab. Look under the 'CPU' column. 'System Idle Process' should probably be up in the 90s, with everything else showing either '00' or a very low figure (such as '02' or '04'). If anything there (other than System Idle Process) is showing a high figure, that's the cause of the problem.
If you don't recognise the name of the process, post again (telling us what it shows up as) and we'll try to help.
Chris