How to do this on an iPhone?

Okay so I have an iPhone 4. I've seen my friends do this thing with their iPhones, and I want to know how to do it too. They somehow press something on their phone, making a minus sign come up in the corner of their apps. I asked them why they did that, and they said it saves the battery on twit phone. So how do I do that? This is different from deleting an app, there's an x tht comes in the corner of the app, but in this case, there's a minus sign. How do I do that?

They jailbreaked it, BUT you can also do it by double pressing your home button

Go to home screen where message, settings, camera etc icons are and press the home button on the phone twice, then a list of apps will come up at the bottom, just hold your finger on any one of them then minus signs will appear

So… You could ask your friends WHY they did that, but you couldn't ask HOW?