IRPs are the main objects that represent the input (a request) and the output (a response) from a device. In many cases, a request packet is simplified by a chain of drivers until the message is understandable by either the final device or the user-mode application (depending on the direction).
For example, consider that you want to play a music file (such as an MP3 file). Once the file has been opened by an application that understands MP3 format, it is converted into something that can be understood by a kernel-mode driver. Then, this driver simplifies it for the next driver and so on, until it reaches the actual speaker as an encoded group of waves. Another example is an electric signal from a keyboard, which is simplified to be a click on a button using an ID (for example, the r button). Then, it is passed to a keyboard driver, which understands that this is the letter r and passes it to the next one. This continues until it reaches, say, a text editor, such as Notepad...
