When playing Dungeons and Dragons, you’re going to be rolling a lot of dice. The dice are the d20, d12, d10, d8, d6, d4 and the d100. The number that comes after the “d” is how many sides there are on that dice. If you’re looking at a spell or a weapons damage dice, you might see that it has a number before the “d”, this means how many dice you roll, so attacking with a greatsword is 2d6, so you take two 6 sided dice and roll, or roll the same dice twice!
The d20 is probably the most important part of D&D, whenever you want to do something such as persuading an NPC to let you pass, or if you want to look around your surroundings to even attacking, you roll a d20. The higher the number, the more successful the action you wanted to do, However if you roll low, the more of a failure the action is. Now rolling a nat 20, is usually the best thing you can roll! However, it doesn’t always mean it’s a complete success, you can still fail. If you’re talking to a king and try and persuade him to give you his crown and roll a nat 20, he still isn’t going to give you his crown. Now it’s possible to get over 20, hell, sometimes you can get 30 and over! But still that doesn’t make the impossible possible! Rolling a natural 1, usually means complete failure and can have consequences, say you’re aiming down your crossbow, your target is distracted by one of your party members who is hacking at it, you fire and roll a nat 1. Cut to your party member, they might see a crossbow bolt flying towards them, they’ll have to roll their dice to try and dodge out of the way of your bolt.
The d12 is a Dodecahedron with 12 equilateral hexagon faces. The d12 works well as a random hour generator. It is also used for heavy weapons like the great-axe or some magic weapons damage. The DM may also use this die to determine the month or other measurements. The d12 is mostly used for high damage weapons and spells. A simple great-axe is an example of a weapon that uses a d12 for its damage dice. Poison Spray is a cantrip that does d12 damage. In 5th edition, Barbarians have a d12 hit die.
The 10 sided dice is commonly used in dungeons and dragons. This particular die is used for large damage rolls and certain spells. The d10 is known as the pentagonal trapezohedron, each face is a kite and the die consists of two sharp corners. Much like the d12, the d10 is primarily used to determine damage from specific weapons or spells. Weapons like a Halberd and a Heavy Crossbow use a d10, meanwhile it is used for spells like Fire Bolt and Inflict Wounds, you won't need a lot of d10 early on. They'll come into play more at higher levels.
The eight-sided octahedron which looks like two pyramids attached the base. This die is most commonly used when calculating damage for light to medium-sized weapons. When figuring out how many hit points a Cleric receives during a level up, you would use a d8. The DM may have other uses for the d8 as well.
The standard cube-shaped dice, the d6 is used in several ways, from weapon and magic damage to recovering hit points during short rest, hit point level up rolls, and sometimes used for rolling a d3 or to assign targets. It is also used when creating characters and determining stats. Multiple d6 rolls can be used in place of a d12 to reduce critical strikes or failure as with the greatsword or maul weapons.
Also called a four-sided die, the caltrop, the painful one. It is a tetrahedron with 4 equilateral triangle faces, it always lands face up, and really doesn't roll that well. This dice is numbered 1-4 and we read the top number or the number that is face up, as there are some variations where the upright number is on the bottom. There is also a variation where the die does not have this caltrop shape, as with this set of magician's dice. While not often used, you may use a d4 when calculating damage from small weapons like daggers. You can roll a d4 in place of the normal damage of your unarmed strike or monk weapon as a bonus action. The DM may have other uses for this die as well when calculating chance. If there are 4 players, the DM may use the d4 to assign targets.
A d100 is typically two pentagonal trapezohedron, using the d10 as one of them. The second pentagonal trapezohedron counts up in 10s, 10,20,30 etc. So when you roll these two dice, you might get a 60 on one dice and a 7 on the other. This results in a 67. The main use that a d100 has is to determine percentages, whether for save rolls or for spells-to-hit. It can depend on the edition of D&D that you play, as well. For example, you'd use a d100 die when determining whether a sorcerer is able to cast their spell correctly or not.