We classify faults by how the two rocky blocks on either side of a fault move relative to each other. The one you see here is a reverse fault. Take a look at the side that shows the fault and arrows indicating movement. See the block farthest to the right that is shaped kind of like a foot? Thats the foot wall. Now look at the block on the other side of the fault. See how its resting or hanging on top of the footwall block? Thats the hanging wall.
Heres another way to think of it: the hanging wall block is always above the fault plane, while the foot wall block is always below the fault plane.