What does jailbreaking an apple product mean?

I have heard about people who jailbreak their iphones or ipads or ipods and i have always wondered what that means as i don't have an iphone just an ipod which i'm pretty happy with. Just curious

Added (1). Thanks but how does it break it? Does it load a virus on it or can you not load it anymore. I also have heard of peoples apple products stop working after awhile but i never understood how exactly

Http://en.wikipedia.org/wiki/IOS_jailbreaking

Not all iPods can be jailbroken - that's basically for IOS devices - iPod Touch, iPad and the iPhone.

It means that you use a program which is legal by the way which let's you download more programs. Example: installous, you can get free apps from it which are from the appstore and example 2: winterboard, gets you a lot of themes and more, I advise you to jailbrake your ipod touch because its awesome dude

It lets you do all the things that you can't do with the iPod that apple gives you. So you can change the color/text of slide to unlock, battery color, apps, lettering. And you can also download free movies and music! It's cool, but I've heard they crash down very fast. I haven't done it yet, but I've been considering to one of these days!

It means you can get free apps, games, you can jailbreak it with redsnow or greenpoison.