Most keyboards feature 3 rows: a top row, a middle, or "home", row, and a bottom row.
The most common keys are usually placed on the home row. Since your fingers rest on the home row, it requires the least amount of movement.
Most letters cannot fit on the home row, however, and thus require you to move your fingers off of the home row.
To reach a key on the top or bottom row, you must move your finger one key's worth of distance - or potentially more if you're using a row staggered keyboard, or moving your index or pinky fingers horizontally rather than just vertically.
However, it is possible to require zero movement to press home row keys, and only half a key of movement to reach the top and bottom row.
If you treat pressing the top and bottom row keys on the same column as a pseudo-home row.
My keyboard layout, delusional, uses a chorded home row.