Easy explanation - the world is spinning round, so the equator gets thrown outwards slightly, like people spinning balls of pizza dough into flat pizza bases.
Or maybe not.
Here's my gloriously long winded explanation...
As top telly scientist Prof. Jim Al-Khalili, explained in his programme "Gravity and me":
Rule 1. There's not really such a thing as gravity. Time runs more slowly near large masses and smaller masses want to move to where time moves more slowly.
Rule 2. Time moves more slowly for fast moving objects.
So with GPS satellites, they have to make two adjustments - the clocks on the satellites seem to be running a bit faster than clocks on earth because they are further away from the mass of the planet; but the satellites are moving quickly, which means clocks on satellites seem to be running a bit slower than clocks on earth.
The prof realised (after some false starts to which he cheerfully 'fesses up) that the same applies if you compare a clock at the North Pole (nearer center of earth but not rotating) with a clock at the equator (further away from center of earth but moving at 1,000 mph). And - unlike for satellites - these two effects exactly cancel out!
This is hardly surprising, really. If we consider the earth to be a large blob of slow moving liquid (and ignore the thin layer of rocks floating on top), it must be clear that if a drop anywhere on the surface of the blob can move to somewhere where time is passing more slowly, it would so. (This is no different to considering a liquid that has been poured onto a flat surface). So we can safely assume that four billion years later, the clocks for all drops on the surface of the liquid part of earth are moving at the same speed.
That is ultimately why the earth bulges at the equator - if you started with a perfect sphere, a clock at the equator would run more slowly than a clock at the North Pole (nearer the center of the earth and moving quickly). So liquid on the surface flows from the Poles towards the equator until the equilibrium is reached.
All part of the service!
Debate Magazine
