Information Architecture (IA) is one of the most understated arts (yes, I mean arts) of web development. Most people do not know what it is. If they have heard of it, they likely recognise that it is somehow important. There is a lot of myth around what it is, with good reasons. It has a wonderfully strange history.
In a nutshell, IA are the principles you use to structure all of the information you manage. Some of it might not even exist within your website. It can include customer information, product information, as well as any documents and media.
The purpose of considering how all of your information works together is the key to making sure your business online aligns with your business goals.