Advantage and disadvantage in Baldur's Gate 3 explained

One thing to note is that advantage or disadvantage on a passive check (that is, a check you wouldn't roll for) are conveyed through a +5 or -5 respectively. For example, if your passive perception score is normally 13, with disadvantage from dim lighting it would be an 8. A hidden enemy would only need to roll an 8 or above on their stealth roll to avoid notice.