Minimum Roll Threshold
Player's Handbook · Using Ability Scores
Certain features, such as the Great Weapon fighting style, add a minimum roll threshold, which means your dice can no longer roll below a certain amount, instead treating the die's result as the threshold. This threshold varies depending on the die size: 2 for d4, 2 for d6, 3 for d8, 4 for d10, 5 for d12, and 8 for d20.
For instance, a character with the Great Weapon fight style wielding a two-handed weapon that deals 1d12 rolls a 3 on a damage die, they treat the result as if they'd rolled a 5.