In "The Wall," what does the narrator mean when he says: "I had spent my time counterfeiting eternity, I had understood nothing."?

This is a story that is all about the psychological
process of facing your own death and the kind of thoughts and emotions that go through
your mind at such a time. The narrator and his two friends are due to face the firing
squad the next day, and we are privileged observers of what goes on in the narrator's
mind and what he observes happening with his two friends. As such, this story is about
ultimate realities and how we can face them to the best of our
ability.

The quote you have selected is very pertinent when
we bear this in mind. Let us just remind ourselves of the meaning of the word
"counterfeit." In this context, counterfeit means to carry on a deception or to
dissemble. This refers to one possible psychological reaction that man can have when
faced with death. The narrator here expresses that he has spend his time trying to trick
or deceive eternity or death. However, at this stage in this excellent short story, he
realises how in vain that attempt was and how he now must face eternity head on without
any more attempts to avoid or get away from death. This quote therefore represents an
acceptance of the narrator's fate.