What is the meaning of the noble Hadith: "Whoever conceals [the fault of] a Muslim, Allah will conceal his [fault] on the Day of Resurrection," and does it apply to those whom I have seen committing reprehensible acts? Should I conceal their actions or expose them?

Whoever conceals a Muslim—meaning their body or their flaws—by not backbiting them and by defending them, Allah will conceal them in this world and the Hereafter. This applies specifically to those not known for corruption. As for one who is known for it, it is recommended to report their matter to the ruler. And whoever becomes aware of someone committing evil deeds, they should advise them and conceal them if they are not openly defiant. Concealment does not mean practicing evil.