It's all about the reader, yo. Reader-Response Theory had a big following, and there were a lot of theoretical spinoffs from it. Readers of a book are part of a great big dynamic process; they're in a relationship with the book that makes meaning and makes the book meaningful. The reader rescues the book from its doorstop existence.
I'm in good company with other Reader-Response people (like Roland Barthes and Stanley Fish), but the point is that we all seriously question the ideas of New Historicism and New Criticism, which both completely diss the reader.
We Reader-Response critics care way more about the reader's experience than about the author's intentions, the text's structure, or the context from which the book emerged. Sorry, Flaubert—much as we love Sentimental Education, we don't care that you wrote it while things were falling apart in the French government (again). We care how people feel when they read it.
Not to brag, but this baby is all mine. That's right: I cooked up the term "Horizon of Expectation" all by myself.
Here's what I'm getting at. You have a book written at a certain place and at a certain time. Then you have a reader, who probably exists in a completely different place and time. Let's say that reader lives in Berkeley in the 1970s and is fully into the whole hippie thing. That reader reads like someone in Berkeley during the 1970s, which is different from the way someone in Paris in the 1920s would read, and it's different from the way someone in Berlin in the 1950s would read, and so on.
The horizon (or outlook) changes for every reader precisely because history changes how readers of each era experience books.
Imagine a triangle with a reader, a text, and the text's author at each angle. As I hope I have made clear, not all readers have the same experience with a text. I mean, I love As I Lay Dying, but maybe you don't. That's one way in which we're different.
On top of that, every new book we read adds to our general understanding of books—and especially of genres. So, because I'm a bookworm, I expect an epic to end with a grand finish, a tragedy to conclude on a sad note, and a comedy to end with a marriage (if we're reading Shakespeare, that is). In our experience with a new book, we reflect on our past expectations and modify our future ones. Different readers bring different expectations based on how many other books they've read.
Simply put, with new data, we are forced to rethink our assumptions. That's what I call the "horizon of change." It's like a gauge of how much you, the reader, will change by reading a certain book in a certain context.
By the way, when you read something that does not challenge you to reconsider your expectations, you have a very small horizon of change—and perhaps no horizon of change at all.
I like to call this baby by its German name, Wirkungsgeschicht, but let's keep things simple. This concept is all about how history "impacts" or changes or affects the way we read a book. You know how important the reader's perspective is to us Reader-Response folks, so with this concept I zero in on how readers always consume books through the lens of the historical moment in which they live.
No piece of literature is "timeless" in the sense that it is always understood by all readers across time and space in the same way. A reader must bring his or her own historical context to the proverbial table.
This here's another way of discussing the importance of the reader in relation to the text. (If you're curious, you can read all about it in my essay "Literary History as a Challenge to Literary Theory.") Marxists go on and on about the importance of history in the creation of art, and Formalists talk about art like it appeared out of nowhere.
I argue that the reader experiences his or her own aesthetics when reading a work. I make every effort humanly possible to make sure that we don't forget about that fine body of people called the readers, who make meaning by actively participating with the book.