Simile - use of something from one field to talk about another, using a direct comparison - as / like, for example "bright as a sun".
Dreams give us hope, purpose, motivation. Comparing his childhood dream to the sun adds another layer of meaning:
The wall - a metaphor for obstacles, anything that prevents the speaker from realising his dream. For example, racism, internal (mental) limitations such as laziness and loss of hope (from being discouraged repeatedly), and physical limitations. The wall grew higher (the obstacles mounted) until the speaker couldn't see the sun (his dream) anymore, and he gives up. The wall blocks the lights, and he feels overshadowed, in the dark.
black - darkness | white - light |
---|---|
evil | good |
death | angels - pure, heavenly |
shadow | light |
The speaker in the poem says he is a shadow. A shadow has no real existence, and it can't even be seen unless there is a light and a solid object. By calling himself a shadow, the speaker expresses his feelings of being marginalized and having no real existence. He also notes that he is black. This can be literal (because he is black or because of the absence of light) or figurative - lesser, bad.
At the point that the speaker can no longer see his dream, he lies down. This expresses his powerlessness, and his loss of hope. When cut off from his dream, he gives in and gives up. The speaker can no longer see anything other than the darkness. He can't even imagine hope.
At the end of the poem, the speaker realizes that to overcome the limitation, the obstacle that prevents him from reaching his dream, he must do it himself and use his own hands. His black hands will shatter the darkness and break down the wall. He concludes with hope and the belief that his hands can break the wall.
The poem represents a process, and we can distinguish three time frames, each with a different tone: