As you might already know, iOS is the operating system that runs on iPhones, iPod Touches and iPads. The first iOS device was the iPhone and was released in January, 2007. Back then the operating system was called "iPhone OS". Since that time more devices were released running iPhone OS than just the iPhone. This is why a while ago Apple announced that the operating system would from that point on be called "iOS" instead.
When you’re developing an app for iOS you most probably are focusing on just one specific device with a specific version of the operating system. For example you’re developing for iPhones running iOS 4.2. If your only target audience is iPhone users then this is good enough. But you could make you’re app support other devices and iOS versions as well. This would extend the target audience for your app. As it turns out, it isn't much of a hassle to do so!