For ELI5, think of it this way. The earth is spinning, and at the equator it's moving really fast, like 900 mph. At the north pole, the earth isn't moving at all, so in the northern hemisphere, you can picture all land to the north of you as moving slower than you, and land to the south as moving faster than you. If you run south, it would spin you because it's moving 'sideways' faster than you are. Cross the equator and suddenly it gets slower as you run south, literally putting the brakes on your spin.