The more you play BG3, the more you will realize how important it is to get advantage on your attacks and avoid disadvantage. When you have an attack prepared and are hovering over a target you will see a % to hit. On the bottom left hand corner of screen is a black box with a bit more info. You may see green up arrows indicating advantage on your attack, or red down arrows indicating disadvantage.
Advantage means you will roll to hit twice, and it will use the higher dice roll. Disadvantage means you roll to hit twice, and it will use the lower dice roll. These extra rolls can seriously hurt or help your chance to hit the target accordingly. If you have 5 sources of advantage (or disadvantage) this does not mean you roll 5 times and pick the highest (or lowest) one. You still only roll two dice.
If you have both advantage and disadvantage on the same attack (for example, you are blinded so have disadvantage, but are making a melee attack against a prone enemy so have advantage) then they cancel out. If you have 5 sources of advantage on an attack, and 1 source of disadvantage, then they still cancel out. The opposite applies as well. You don't keep track of "Well there's X sources of advantage which is more than Y sources of disadvantage, so I should have advantage on the attack." That's not how it works. 1 of either is enough to cancel an infinite number of the other.
UPDATE: Spells like Sacred Flame, Acid Splash, Thunderwave, Lightning Bolt, etc. that force your enemy to make a saving throw are NOT attack rolls. For an attack roll you are the one rolling to see if you can get through the enemies defenses. Spells that force the enemy to make a save means that the enemy is rolling to resist the spell effects. While it is possible for your enemy to have advantage/disadvantage on saving throws, this is much more rare than advantage/disadvantage on attack rolls (what this guide is all about).
Sources of advantage
- Attacking an enemy who can't see you (you're hidden, or they are blind)
- Attacking a prone enemy with a melee attack
- Attacking an enemy affected by a status effect like "paralyzed" from the Hold Person spell (this also automatically crits if it hits). Other effects that will give advantage include restrained or being hit by Guiding Bolt
- Not D&D 5e official but in the game: Making a ranged attack on an enemy who you have the high ground on
- Not D&D 5e official but in the game: Making a melee attack against an enemy's back (backstab)
Sources of disadvantage
- Making a ranged attack while an enemy is near you (called "Threatened" in BG3)
- Attacking a prone enemy with a ranged attack
- Attacking somebody affected by a spell like Blur
- Attacking an enemy you can't see (in a fog cloud, darkness, or you are blind)
- Attacking an enemy beyond the normal range of your ranged weapons. For example, a shortbow can attack enemies within 320 feet. But anything beyond 80 feet will have disadvantage
- Using a heavy weapon (greatsword, heavy crossbow, etc.) as a small race (halfling, gnome). Dwarves are NOT a small race.
- Not D&D 5e official but in the game: Attacking an enemy in dim light.
- One good way to get around this is to use a cantrip like dancing lights in the area around your target covered in darkness
- Not D&D 5e official but in the game: Making a ranged attack on an enemy who is higher up than you
There are other sources of advantage and disadvantage in D&D 5e (attacking while prone, sunlight sensitivity, dodge action, etc.) but they are not in BG3 so not worth discussing here.