One Die: The d20 Roll

When your attempt has a real chance of failure, you roll 1d20, add an ability modifier, add your Proficiency Bonus if you're proficient, and compare the total to a target number. Every contested moment in the game — picking a lock, resisting a curse, swinging a mace — is this one roll wearing three uniforms. The deep text is @Missing file.

The three rolls

• Ability check — you attempt something; the GM names an ability and a Difficulty Class. Easy 10, moderate 15, hard 20. Skills like Stealth or Persuasion are just checks you might add proficiency to.
• Saving throw — the world attempts something on you: Dexterity to dodge the fireball, Constitution against poison, Wisdom against a charm. You're proficient in two saves, granted by your class.
• Attack roll — against a creature's Armor Class. A natural 20 always hits and doubles the damage dice; a natural 1 always misses.

Advantage and disadvantage

Circumstances tip the odds: roll two d20s, take the higher with advantage, the lower with disadvantage. Sources never stack — two dice at most — and if you have both at once, they cancel completely. It's the edition's favorite dial, and earning advantage before you roll is most of the tactics in the game.

Proficiency Bonus

One number, set by level — +2 at 1st, climbing to +6 at 17th — applied once per roll to the things your build is trained in: certain skills, two saves, your class's weapons, your spells. @Missing file covers it, including Expertise, which doubles it.

Inspiration

When you play your personality, ideals, bonds, or flaws in a way that makes the game better, the GM can award Inspiration. Spend it to give yourself advantage on one ability check, attack roll, or saving throw. You either have it or you don't — it doesn't stack, so spend it. The details are in @Missing file.

The best roll

As in every edition: the one you never had to make. Describe your approach concretely — the right plan turns a hard check into no check at all.