Information architecture is the practice organizing and arranging information to make it understandable.

If you had your own grocery store, where would you put carrots? Apples? Bread? It sounds pretty simple until you start getting into things like peanuts, jelly, cream cheese, cake mix, and so on.

Information architecture is figuring out how to arrange things into something that can be understood by your user. This includes defining hierarchies, parent-child relationships and in general making sense out of an (informational) messes.