It means all of your memory has been used up, and even the assigned hard drive space (swap) to use as virtual memory has been used up. I'd see what's causing the issue.
Run: ps axfu
And copy and paste the results.
Also post : ps -u nobody , to see is possible perl bots / etc have been started by the apache user.
Alot of Apache processes, either a very busy site, or someone's abusing it and overloading your server on purpose.
Run
httpd stop;killall -9 httpd;httpd startssl
That should restart and clear up apache processes. Just a band aid solution for now.
Let me know if it starts to overload again, and Ill post directions on seeing if you have a possible DoS, or if not, we can tweak your script and or possibly Apache.