What are the flaws/defects that the Prophet (peace be upon him) meant in his saying: "Whoever conceals the fault of his brother, Allah will conceal his fault on the Day of Resurrection"?

Indeed, spreading indecency about believers is religiously forbidden and carries a painful punishment in this world and the hereafter. The Prophet forbade harming the servants of Allah and seeking their faults, and warned that whoever seeks to tarnish a Muslim's reputation with words that absolve the Muslim will be cast into the Fire by Allah. As for one who conceals his Muslim brother's faults, Allah will conceal his faults on the Day of Resurrection.