Hit Dice, or Hit Die, also known as HD, which you'll see mentioned in class kit descriptions as well as in some spells.
For non-player creatures, HD is similarly referred to as their levels, with higher HD often representing stronger and tougher creatures.
For playable characters, it’s one of the defining factor in their total Hit Points (HP), implemented in their earlier levels varying among classes.
Single class HP Edit
Priest class group lvl1-9 gain 1d8 + constitution modifier each level, and a fixed 2hp per level afterwards
Wizard class group lvl1-10 gain 1d4 (1d6 for Dragon Disciple) + constitution modifier each level, and a fixed 1hp per level afterwards
Rogue class group lvl1-10 gain 1d6 + constitution modifier each level, and a fixed 2hp per level afterwards
Multi-class HP Edit
Here comes the most complicated part, generally the HP multi-class character gain on each level is divided by 2 or 3, depending on their class number.
To elaborate, for each level advance in their Warrior, Priest group lvl1-9, and Wizard, rogue group lvl1-10, they gain [(Hit Dice of the leveled up class / class number)rounding] + [(highest con modifier of their classes x total level number / class number)rounding] – [total con modifier from previous levels] ; for the levels afterwards, it’s [(fixed hp per level / class number)rounding].
Rounding rule is: divided number greater than 1 rounds to floor, divided number less than 1, including negative numbers, rounds to ceiling.
eg. A con 18 triple multi Fighter (1) Mage (1) Cleric (1) gains [(Fighter HD 10 / 3)rounding + (Mage HD 4 / 3)rounding + (Cleric HD 8 / 3)rounding] + [(Fighter con modifier 4 x 3 / 3)rounding] - [total con modifier from previous levels (0)] = [3 + 1 + 2] +  - = 10 as starting HP.
Then leveled up to F(1)M(1)C(2), the HP gain is [(Cleric HD 8 / 3)rounding] + [(Fighter con modifier 4 x 4 / 3)rounding] - [total con modifier from previous levels (0 + 4)] =  +  –  = 3
Then on F(2)M(1)C(2), gained HP [10 / 3] + [5 x 4 / 3] - [0 + 4 + 1] =  +  -  = 4
Then on F(2)M(2)C(2), gained HP [4 / 3] + [6 x 4 / 3] - [0 + 4 + 1 + 1] =  +  -  = 3
Until starting to gain the fixed HP, which happens to be rounded to 1 per level for all classes.
Dual-class HP Edit
Dual-class characters stop gaining HP once the dual begins until they reactive their first class, then they gain HP each level as the second class normally would.
eg. A 15 constitution Wizard Slayer (7) who stops gaining HP when becomes Wizard Slayer (7) > Mage (1), will start to gain 1d4+1 each level from Wizard Slayer (7) > Mage (8) until Wizard Slayer (7) > Mage (10), then gains a fixed +1 per level as a Mage would.
Quick reference table Edit
|Class group||Hit Dice per level||Max HD level||Fixed HP gain per level||Primary Abilities|
|Barbarian Dwarven Defender||d12||9||3|
|Ranger||d10||9||3||Strength, Dexterity, Wisdom|