According to a different source, the question refers to the speech "I Have a Dream" by Martin Luther King.
The metaphor that King uses in lines 17-31 is that of cashing a check. King tells us that the black people of the country were promised equal rights and he compares this to a check. However, they have not been able to cash it, as the country has refused to give them the rights that they were promised after emancipation.
