The back of your head is a physical place that's opposite from your face, which is at the front of your head. The back of your mind is a figurative place where you keep thoughts about things you intend to start doing at some point in time.

Hello, Yusuke! As stated before, "back of my head" is a literal location on the head and "back of my mind" is only used figuratively. Here are a couple examples of both! 1. The football player was hit on the back of his head with the football and suffered a concussion as a result of it. 2. Despite having enough money to pay the bills, money worries are always in the back of my mind. Hope this helps!