Dr King refers to a check or promissory note that was given to African-Americans what is the promissory note?

In his famous "I Have a Dream" speech, Dr. Martin Luther King Jr. refers to the promissory note as a metaphor for the unfulfilled promises of freedom, equality, and justice made to African Americans by the United States. He argues that the nation has defaulted on this note, failing to deliver on the rights and opportunities guaranteed by the Constitution and the Declaration of Independence. King's call is for America to honor its commitments and ensure that all citizens receive their rightful due.