No dice are rolled, happens passively.
Repeated examples of a check, e.g. "I constantly look for traps."
DM might use passive checks to see if a character passes a check without telling them to roll a die, e.g. do the PCs notice a hidden monster?
*10* + all check modifiers Advantage = +5 Disadvantage = -5
The PC is walking through a dimly lit cave, there's a monster hiding nearby. The PC doesn't have darkvision.
The monster is a goblin (Stealth +6) and the DM rolls a d20 to see if the goblin is stealthy. The DM rolls a 10, with the +6 that's a total of 16.
Base Wisdom = 15 (+2) Proficient in Perception = +2 Base = +10 Disadvantage for dim light = -5 Total = 9
The PC fails to spot the goblin.