The "Rich Young Ruler" is a biblical figure mentioned in the Gospels of Matthew, Mark, and Luke. He is described as a wealthy young man who approaches Jesus to ask how he can inherit eternal life. Jesus tells him to follow the commandments, which the young man claims to have kept since his youth. When Jesus instructs him to sell all his possessions and give to the poor, the young ruler becomes sad because he has great wealth. This story illustrates the challenges of materialism and the difficulty of prioritizing spiritual wealth over earthly riches.