•Why it earns a slot
The poem is a foundational text of English Romanticism that gave the language the phrase 'water, water, every where, nor any drop to drink' and constructed one of literature's most sustained explorations of guilt, penance, and the moral weight of a single destructive act against nature.
An old sailor compels a wedding guest to hear his tale of how he shot a friendly albatross during a voyage to Antarctic waters, bringing supernatural doom upon his entire crew. All two hundred crew members die, while the Mariner is condemned to live, and he is only partially released from his curse when he spontaneously blesses sea creatures he had previously found repulsive. He returns home but is forever compelled to wander and retell his story as penance.
This distillation is written from the freely available original, which is always the better read when you have the time: gutenberg.org.