The hard drive light comes on to show that data is being written to it or read from it. It happens whenever you save a file to a hard drive or when you open one from it. Moreover it also happens when lots of other things happen, such as when your computer updates its anti-virus software. Indeed, there are so many things that go on 'in the background' that it's not unusual to see frequent hard drive activity indicated. (The only time that you need to worry is if you see the light on continuously. That could indicate a problem with the computer).