Your Power Rating influences mission difficulty. Increase it by equipping items with higher Power Rating.
The Power Rating represents the might of your character. It's determined by adding the Power Rating of all of your equipped items together.
The Power Rating is primarily used to compare your character's strength against the Challenge Rating of missions. Depending on which is higher, you gain positive or negative bonuses for the given mission. This mechanic lets you dynamically choose game difficulty for yourself. If you're oriented toward more casual gameplay, accept easier missions with combat bonuses. If you like challenges, embark on missions where the odds are stacked against you. Embarking on more difficult missions will yield slightly better items, while easier missions will yield slightly inferior loot.
To increase your Power Rating, equip more powerful items. To gain more powerful items, you must increase your Inquisitorial Rank. The Power Rating of reward items is directly determined by the Inquisitorial Rank.