[Question] Help Understanding QALYs?

Hi!! Im new to EA and have little to no background in economics/​statistics. Im reading Doing Good Better, and came across this paragraph about QALYs. From what I understand, QALYs measure extending someone’s life or improving it through through lifetime? The paragraph below is super confusing to me. What exactly does “7 QALYs” mean? And the back pain example is super confusing as well.

“The first graph illustrates improving the quality of someone’s life by 20
percent for the sixty years of their life. That would amount to twelve
QALYs in total (60 x 20% = 12). The second graph illustrates extending the
life of someone who is currently at 70 percent health by ten years. That
amounts to seven QALYs overall (10 x 70% = 7). The QALY metric
therefore allows us to measure the size of benefits that different people
receive If you want, you can come up with your own personal quality weights.
Think of an ailment that you have suffered from at some point in your life.
Suppose you suffer from back pain, as I sometimes do. You could think to
yourself: If 10 represents how good my life is when I’m perfectly healthy,
how good is my life on a day when I’m suffering from back pain? This can
be difficult to answer, so a way to make your judgments more precise is to
ask yourself about what sorts of trade-offs you’d make. If you could live
one extra day at perfect health or a certain larger number of days living with
back pain, at what point would you be indifferent? In my own case, I’d be
indifferent between living an extra four days perfectly healthy, or an extra
five days with back pain. That suggests that I think life with back pain is 80
percent as good as life pain-free.”

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