What is the meaning of the حديث: "Whoever shields a Muslim in this world, Allah will shield him on the Day of Resurrection"? And am I obligated to expose the reprehensible acts committed by some young men whom I used to see, in order to protect myself, or should I remain silent and bear the consequences of their actions?

Concealing a Muslim's faults is a great good deed and reward for whoever does it seeking the pleasure of Allah. For whoever conceals a Muslim, Allah will conceal his faults in this world and the Hereafter. However, if the person committing the sin is open about it, or is well-known for what they have done, or if concealing their fault leads to harm, then it is preferable not to conceal it. Instead, their case should be brought before the ruler, unless there is a fear of greater mischief from doing so. This is because, in such a case, concealment encourages the perpetrator to commit more corruption and violate sacred bounds. Therefore, if concealing the faults of those who commit evil would cause you harm, then you are not obligated to conceal them.