What does it mean to dream about coffin?
A coffin usually marks an ending you are facing, the close of a phase or an old self more than a literal death, and sometimes a feeling of being boxed in.
Four ways to read it
Each reading below comes from a named tradition and a cited source, so you can see where it came from and decide what fits your dream.
Freud
Freud would read a fear or wish about death, and the confinement of something the dreamer wishes buried.
After Freud, The Interpretation of Dreams (Brill trans., 1913), public domain.Jung
Jung would see the burial of an old attitude so renewal can follow, the container of what must end before it can change.
Jungian concept (our summary).Modern
Continuity ties coffins to waking grief, endings, and at times to feeling trapped or shut away.
Continuity hypothesis (our summary).Mythological
The coffin carried Osiris down the Nile to his death and return, the container of the god who dies and rises, ending bound to rebirth.
Rosmerta Publishing editorial synthesis (after Bulfinch; Frazer).