King used a financial metaphor to refer to the lack of compliance with the civil rights that the Declaration of Independence and the Reconstruction Amendments to the US Constitution (14th and 15th) included for all US citizens, without discrimination in terms of race.
King aimed to express that the US still owed, to its black citizens, the defaulted "promissory note" that Martin Luther King Jr. had mentioned and demanded in his remarkable "I have a Dream" speech, delivered in 1963. Instead of guaranteeing the promised rights of life, of freedom, and of the pursuit of happiness, black people had received a black check, one with no funds on it and were still waiting for the payment of such debt.