Whenever a character in your party in Baldur's Gate 3 notices a trap or points to the origins of a religious shrine that they carelessly pass while walking around, they take a passive skill check. Passive skill check occurs for the whole group when they cross paths with something they may need to use their skills to notice it.
These passive skill checks occur throughout your journey. They happen when your characters pass a specific object and a small pop-up appears above their head that shows the skill checks they are performing. Usually these are Survival, Perception, Investigation, Insight, Arcane, History, and many more. Whenever you see this icon appear, you can expect to see a 20-sided die (a d20) appear below. A progress bar revolves around the d20, and at the end of it you'll see if your characters passed or failed their passive skill test. A passive skill check is different from the one you choose to perform in a conversation.
When a character passes the passive skill test, they notify the party (targeting you) of what they were looking at. This could include the identity of a cave they were passing through, letting you know what animal resides there, what god and religion a particular statue comes from, or if they notice there is a secret passage. If all of your characters fail their checks, they won't see what's there, preventing you from gaining additional knowledge. This can prevent you from learning how to access a secret area, or even prevent you from knowing a trapped chest that your thief is about to unlock.
Each character in your party has a chance to roll a passive skill check, even if they're not good at it. For example, your thug will attempt to take a religious skill check at an altar they pass, even if they have a negative score of two with religion.
Whenever you see a skill name pop-up above your character, stop for a second to see what it might refer to. You might learn about a nearby statue or notice a trigger wire hinting at a nearby ambush that your party must avoid.
