If your app gets a memory warning (such as in your view controller’s didReceiveMemoryWarning method) you need to release any non-critical data. Anything that you’re using that cached, for example, or that can be regenerated, should be dumped.
For example, if your app crunches some numbers and stores the result in a big array, if you’re not actively using that array, you should release it. Then, regenerate it when you need it again.
A little more information is here:
Observing Low-Memory Warnings