How does passive perception work
Total noob question, but the passive perception is confusing. I don't understand how to calculate it or use it, and I need help.
Actually there are three questions - how to calculate passive perception (as a player for your charsheet), how to use it (as a DM) and how does it work in general (this one is too broad question for the SE format).
If you look over at the 'Related Questions' on the right, There are a ton of pre-existing questions that probably answer what you're looking for.
Per Passive Checks (PHB page 175, emphasis mine),
A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.
Here’s how to determine a character’s total for a passive check:
10 + all modifiers that normally apply to the check
If the character has advantage on the check, add 5. For disadvantage, subtract 5. The game refers to a passive check total as a score.
For example, if a 1st-level character has a Wisdom of 15 and proficiency in Perception, he or she has a passive Wisdom (Perception) score of 14
So 10 + 2 (from proficiency bonus) + 2 (from a 15 Wisdom Ability Score)
Per Wisdom, Perception (PHB page 178),
Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses. For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest. Or you might try to spot things that are obscured or easy to miss, whether they are orcs lying in ambush on a road, thugs hiding in the shadows of an alley, or candlelight under a closed secret door.