It's basically when ur system seems to stop in the middle of a task and everything grinds to a halt...if you hit Ctrl+Alt+Delete to open the Task Manager, you can check the 'Applications' tab to see what processes are running and which are not responding...in other words identify the culprit.
Click to highlight any process that says 'Not responding' and then click the End Task button. This should get things moving again.
Try closing everything and then just opening that one offending application by itself and see if it still hangs the pc.
If the problem only occurs when u try to open multiple apps, then u might need more memory. Alternatively, if the app alone fails to run, it could be a bug. There's loads of things it could be...software or hardware-related, so if any of the above fails to work, come back and tell us what the application is that seems to be causing the problem, what ur OS is, how much RAM u have and CPU speed....etc.