The main idea of this poem is a moral message, warning against thoughtless and foolhardy actions such as the one the Mariner performs in shooting the albatross. The albatross had done the Mariner no harm at all; in fact it seems it had helped to guide his ship, so that his wanton killing of it appears even more inexcusable.
The poem shows the terrible consequences of such a foolish and cruel act, when the Mariner loses his crew and finds himself alone and doomed to wander the earth, telling people like the young Wedding Guest his woeful story and warning them to always think before they act.
A subsidiary theme of the poem is that one should respect nature and all living things. The Mariner never gives any reason for shooting the albatross;it seems as if he does so simply because he has the power to do so. However, the swift vengeance that follows shows how wrong it is to break man’s communion with nature in this fashion. The Mariner sums up this lesson as follows:
He prayeth best, who loveth best
All things great and small;
For the dear God who loveth us,
He made and loveth all. (614-617)
This quote takes on a distinctly religious aspect, directly referring to God and conjuring up a picture of a joyful, harmonious world of universal love. The Mariner has suffered grievously in going against this ideal and serves as a living example of folly.
The central message of the poem, then, is quite a simple one, but it is dressed up in the memorable guise of a vivid supernatural tale which has helped to ensure its popularity to this day.