The equator is the part of the world which, at sea level, is farthest from the axis of the Earth's spin. Since the speed of a point travelling in a circle is proportional to the distance from the centre of the circle, this means there's more centrifugal force (or centrifugal effect) than elsewhere on the planet. Launch points for rockets to space are usually near the equator to take advantage of this effect.
The equator is to the spin of the Earth's day as the ecliptic is to the orbit of the Earth's year.