Seeking an explanation of the hadith: "Fear the prayer of the oppressed, even if he is a disbeliever, for there is no barrier (between it and Allah).”

Please can anyone explain the following hadith? > The Messenger of Allah (blessings and peace of Allah be upon him) said: “Fear the prayer of the oppressed, even if he is a disbeliever, for there is no barrier (between it and Allah).” (It is referenced on e.g. Islam Q&A .)