Triage and Detritus
What you watch
What not to see
What you listen to
What not to hear
What you keep
What to discard
Note: There is as much in the title as in the poem.
Triage is from the French trier, to sort. It is best known as a medical term, the sorting of casualties according to severity of injuries. This is how it was used in the Napoleonic wars, later in World War I, and to this day in emergency medicine. But it has for centuries had non-medical uses, as in the sorting of wool, coffee beans, and even recently in discussing which endangered species to try to save first. So in general, triage is an assessment and sorting according to quality.
Detritus is also from the French. It is the disintegrated material and debris that remains after wearing away, from rocks or from organisms. It is related to the word detriment.
The context, particularly of the last lines (What you keep/Tells you/What to discard), is a review of stuff to be kept or let go. If you look at a thing, you may have all sorts of thoughts—arguments with yourself—about what to keep. But if you look at what is in the absolutely-must-have box, those things that require no thought, that tells you something, maybe everything, about the rest. The essential speaks for itself, but you do have to shut up and listen.