The data is accessed through an interface, a set of functions associated with the data structure. You don't dig through internal memory structures to get the data - you call these functions. That is what "hidden" means in this context.

A better example might be a linked list. With information hiding, you manipulate the list through functions like push and pop. Without information hiding, you manipulate the pointers directly. For instance, I've worked on C codebases where linked list manipulation isn't even hidden behind macros.