These usually represent the character’s basic ability to do something without really being focussed on it or in reaction to something. To roll a passive check, the character must roll the number of dice equal to the relevant ability (such as toughness). These are successful if they achieve 6 or higher. Passive checks can explode. The most common examples of these are the passive awareness and passive toughness checks. Passive checks never take away from the dice pool of a character. In certain circumstances a character can add focus dice to their passive checks but this is at the GM’s discretion and should normally be limited to a maximum of double their original dice (which is a default rule for all checks).
For example, Pietre is making a passive toughness check to avoid throwing up at a horrific sight. However, he is aware that he might throw up and focusses on avoiding this. His toughness is three so he is able to add three dice to this check (to a maximum of six).
As a general rule, when the character wouldn’t be aware, when they weren’t able to focus on something or when they’ve spent their dice and/or focussed on something else then they aren’t able to add active dice to a passive check.