I think it is a special story, in the beginning, we can learn that Mr.Li
is a man who earned lots of money, and he cares about his mother, He
knows she always wants to see the place his father once worked, so he
bought a pair of binoculars to her, boyishly eager to show her the
present, which can make her see things clearer.
But the old lady just can't see anything through them because of her cataracts, then why she says she can?
my opinions, Mr.Li's mother miss the good old time a lot, while Mr.Li
can't remember so much about that, he tried to figure it out but get
nothing, and before he can have the chance to ask his mother, she has
already passed away. The key mentioned in the end probably is the
metaphor for regret of Mr.Li.