The Crit Rule

When a player rolls a natural 20 on their attack dice (also known as a "nat 20"), they will receive the maximum possible outcome of their damage dice in addition to whatever they roll.

For example, if a player were to roll a nat 20 with a longsword, which deals 1d8 damage, they would roll the 1d8 and add the maximum possible damage of 8 to their roll. So if they rolled a 5 on the 1d8, they would deal a total of 13 damage (5 + 8) on their critical hit.