Bonuses

When attempting a check that involves something you have some training in, you also add your proficiency bonus. This bonus depends on your proficiency rank: untrained, trained, expert, master, or legendary. If you're untrained, your bonus is +0—you must rely on raw talent and any situational bonuses. Otherwise, the bonus equals your character's level plus a certain amount depending on your rank. If your proficiency rank is trained, this bonus is equal to your level + 2, and higher proficiency ranks further increase the amount you add to your level.

There are three other types of bonus that frequently appear: circumstance bonuses, item bonuses, and status bonuses. If you have different types of bonuses that would apply to the same roll, you'll add them all. But if you have multiple bonuses of the same type, you can use only the highest bonus on a given roll—they aren't cumulative. For instance, if you have both a proficiency bonus and an item bonus, you add both to your d20 result, but if you have two item bonuses, you add only the higher of the two.
Circumstance bonuses involve the situation you find yourself in when attempting a check. For instance, using Raise a Shield with a compact shield grants you a +1 circumstance bonus to AC. Being behind cover grants you a +2 circumstance bonus to AC.
Item bonuses are granted by some item that you are wearing or using, either mundane or magical. For example, armor gives you an item bonus to AC, while a tactical holoskin grants you an item bonus to Impersonate someone using the Deception skill while wearing it.
Status bonuses typically come from spells, other magical effects, or something applying a helpful, often temporary, condition to you. For instance, the akashic download spell grants a +1 status bonus to a chosen Recall Knowledge check.