The three main rolls of the game—the ability check, the saving throw, and the attack roll—rely on the six abilities. The basic rule behind these rolls is: roll a d20, add one of the six abilities, and compare the total to a target number.
Proficiency is a simple way of assessing your character’s general level of training and aptitude for a given task. It is broken into three different aptitude levels: untrained, capable, and proficient. Each level grants a different proficiency bonus.
Capable. +half your proficiency bonus (rounded down)
Proficient. +your proficiency bonus
Characters have a proficiency bonus determined by level. Monsters also have this bonus, which is incorporated in their stat blocks. The bonus is used in the rules on ability checks, saving throws, and attack rolls.
Your proficiency bonus can’t be added to a single die roll or other number more than once. For example, if two different rules say you can add your proficiency bonus to a Will saving throw, you nevertheless add the bonus only once when you make the save.
Occasionally, your proficiency bonus might be divided (halved, for example) before you apply it. If a circumstance suggests that your proficiency bonus applies more than once to the same roll, you still add it only once and divide it only once.
Advantage and Disadvantage
Sometimes a special ability or spell tells you that you have advantage or disadvantage on an ability check, a saving throw, or an attack roll. When that happens, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage. For example, if you have disadvantage and roll a 17 and a 5, you use the 5. If you instead have advantage and roll those numbers, you use the 17.
If multiple situations affect a roll and each one grants advantage or imposes disadvantage on it, each advantage/disadvantage pair cancels out. After resolving, if you have multiple advantages or multiple disadvantages you roll one additional dice for each advantage or disadvantage. For example, two advantage and no disadvantage on an attack would roll 3d20 and keep the highest 1.
When you have advantage or disadvantage and something in the game, such as the halfling’s Lucky trait, lets you reroll the d20, you can reroll or replace only one of the dice. You choose which one. For example, if a halfling has advantage or disadvantage on an ability check and rolls a 1 and a 13, the halfling could use the Lucky trait to reroll the 1.
You usually gain advantage or disadvantage through the use of special abilities, actions, or spells. Inspiration can also give a character advantage. The GM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.