Hit Dice represent a creature's general level of power and skill.
As a creature gains levels, it gains additional Hit Dice.
Monsters, on the other hand, gain racial Hit Dice, which represent the monster's general prowess and ability.
Hit Dice are represented by the number the creature possesses followed by a type of die, such as “3d8.”
This value is used to determine a creature's total hit points.
In this example, the creature has 3 Hit Dice.
When rolling for this creature's hit points, you would roll a d8 three times and add the results together, along with other modifiers.