Auden's poem makes a point that others are indifferent to suffering or tragedy that does not directly involve them. Despite his father's warning, Icarus flies too near to the sun with wings made of feathers and wax, the sun melts the wax, and Icarus plunges to his death in the sea. In the Brueghel painting that Auden describes, ordinary people with tasks to complete go about their business, only peripherally noticing the tragic death of this young man. As Auden puts it:
About suffering they were never wrong,
The old Masters: how well they understood
Its human position: how it takes place
While someone else is eating or opening a window or just walking dully along
The poem describes how children skate on a pond as Icarus falls because they know life will continue and how the man plowing goes about his business—perhaps hearing the splash or the cry of the boy but not considering it important because he has more immediate concerns to occupy him. Finally, the people on the ship floating by might have seen the "amazing" sight of a boy falling out of the sky, but the boat has to get to its destination and so sails on. We are all usually too focused on the immediacy of own lives, says the poem, to be present for the tragedy of another.
The second stanza of this excellent poem is where you will find the answer to your question. In this stanza, the speaker, who we imagine strolling through a museum leisurely looking at a variety of masterpieces, contemplates suffering and the indifference with which it is often met. He uses one painting in particular to demonstrate his point: Bruegel's Icarus, which depicts Icarus falling out of the sky having soared too close to the sun. He plummets to his death whilst those around him remain either unaware or choose to ignore his death. The poem says that "everything turns away / Quite leisurely from the disaster." Although the "plowman may / Have heard the splash" it was not "an important failure" for him. However, both the plowman and the ship had more pressing business to complete:
...and the expensive delicate ship that must have seen
Something amazing, a boy falling out of the sky,
Had somewhere to get to and sailed calmly on.
Thus the profound indifference to human suffering is characterised by this painting and the way that the death of Icarus is variously ignored or not noticed.