I found the Wikipedia page pretty useful and approachable as an introduction, it has a graphical representation of the graph in @ryancbriggs answer, similar to the ones in the book.
The brown area is the effect of the intervention in QALYs
I think that the quote about back pain refers to the Time trade-off methodology of computing QALYs.
For more details on how QALYs are computed in practice, you might be interested in this recent question and the many links in the answers and comments.
The definition on the forum topic page could also be useful, or some of the posts linked there.
I found the Wikipedia page pretty useful and approachable as an introduction, it has a graphical representation of the graph in @ryancbriggs answer, similar to the ones in the book.
I think that the quote about back pain refers to the Time trade-off methodology of computing QALYs.
For more details on how QALYs are computed in practice, you might be interested in this recent question and the many links in the answers and comments.
The definition on the forum topic page could also be useful, or some of the posts linked there.