The word you already know is the comparative of "young": "My sister is younger than me." But the sense being explained here is different and much narrower. "The Younger" is a label put after a famous person's name to show which of two people with the SAME name you mean — the later one, usually the son or daughter. The earlier one gets "the Elder". So Pliny the Elder was the uncle, Pliny the Younger the nephew who wrote about Vesuvius; Pitt the Younger was the son of Pitt the Elder, both British prime ministers. You meet it in history books, art history and old writing, not in daily conversation. It is capitalised and comes after the name, with "the": Holbein the Younger. Notice English handles this differently from names today — modern English families use "Junior" and "Senior" for father and son (Martin Luther King Jr.), while "the Younger" and "the Elder" belong to earlier centuries and to translated classical names. It is also a family surname in Britain, especially in Scotland and northern England, first given to tell apart two men in a village with the same first name.