The d20
The d20 is used for many uncertain outcomes. Roll it, add the relevant bonus from your character sheet, and compare the total to a target number or a creature's armor class.
Volume One
Dice and Rolls
How d20 rolls, advantage, disadvantage, damage dice, and checks usually work.
Advantage and disadvantage
Advantage means roll two d20s and use the higher result. Disadvantage means roll two d20s and use the lower result.
Other dice
d4, d6, d8, d10, and d12 are often used for damage, healing, random tables, or class features.