Passive perception is basically a measure of how situationally aware you are.
Passive Perception is what game masters use to see if you naturally spot stealthed creatures or something hidden without your character searching the area.
Formula to calculate passive perception.
The proficiency bonus is only added if any.
A wisdom modifier or an ability’s modifier is usually the part of the ability that is used for attack rolls, defenses, skill checks and other features of your character.
The proficiency bonus represents your experience from a role playing viewpoint, and your power curve from a game design viewpoint.
Suppose you are at +15 wisdom modifier, calculate your passive perception.
Therefore, your passive perception is 25.