As you can imagine, there are many motifs in Kurosawa's film, Dreams. Let me offer two of them.
First, there is a motif of human persistence and the ability to succeed in the face of great difficulties. In fact, many of the dreams have an agnostic quality to them. For example, in the dream, "Sunshine in the Rain," the boy transgresses boundaries by looking at the foxes, but he at the end courageously goes to face them to ask for their forgiveness. In the dream, "Blizzard" there is incredible struggle as four mountaineer seek to climb a mountain in horrible weather. A even a demon comes to persuade them to sleep (that is die), but the men continue and persist to the point where they make it. Even in the dream, "Tunnel," there is a sense where the Japaneses officer has to face and overcome the dead by commanding them to go back into the tunnel.
Second, there is also the motif of the horror of war. We see this in the dead soldier and the group of dead soldier in the dream, "Tunnel." We also see this in "The Weeping Demon," where there is a world in which little remains on account of a nuclear holocaust. We also see glimpses of this idea in "Mount Fuji in Red," and "The Village of the Waterfalls." There is also an anti-technology message, which can be linked with the ideas of the horrors of warfare.