To head back means to return to where you came from or to go in the direction you just traveled from. When recess ends and the teacher calls everyone inside, students head back to their classroom. After visiting your grandmother's house, your family heads back home.

You might head back to the library to return a book you forgot, or head back to the kitchen because you left your lunch on the counter.

People use this phrase when they're reversing direction or returning somewhere, whether for a few steps or a long journey. A hiker who reaches the summit will eventually head back down the mountain. If you walk past your friend's house by mistake, you'd turn around and head back to find it. The phrase suggests purposeful movement toward a place you've already been, rather than wandering or exploring somewhere new.