Rolling a 20 on a D20 — also known as a "natural 20" or "nat 20" — is a cause for celebration because it's usually considered an automatic success on whatever task the player's character is attempting. This could be an attack roll, a saving throw, or an ability check. In combat, not only is a natural 20 an automatic hit, but it often also results in a "critical hit", which usually means the player gets to roll extra dice for damage, making their attack significantly more potent. The exact rules for critical hits can vary between different editions of D&D, but they generally always mean good news for the player who rolled the 20. The excitement comes from both the rarity of the event (there's only a 5% chance of rolling a 20 on a D20) and the potential game-changing effects. A well-timed natural 20 can turn the tide of a battle or allow a character to pull off a feat that they would otherwise have a hard time accomplishing. It adds a thrilling element of chance and unpredictability to the game.