When something is overarching, it affects or includes everything. While your novel touches on a lot of subjects and ideas, its overarching theme is the importance of friendship.
When it's used figuratively, this adjective describes a quality that influences every single part of something. You can say that your club's overarching interest is the outdoors, or that the overarching goal of your school is to give everyone a solid STEM education. Overarching also literally means "forming an arch shape over something," like the overarching grape vines you pass beneath to enter your friend's garden.