We might like to make our switchmode power supply's transformer with its core pieces arranged as shown on the left in the following sketch, but for this reason and that, we can't. We need in many cases to provide some kind of break in the core's magnetic path. We need to provide a magnetic gap. Let's call doing that "gapping" if only for lack of a better word.
We might be tempted to do our gapping all in one place as in the center image, but this can be quite a bad idea, especially if the gap needs to be large. Such an arrangement can cause the magnetic flux to undergo a fringing phenomenon which can set up eddy currents in the copper of the surrounding coil(s) which can make the wire get very lossy and very hot.
Think "induction furnace" when contemplating this because that is just how such furnaces are made and we don't want to make a furnace, we want to make a power supply.
What we often need to do is more like in the image on the far right. Gapping is done at all three ferrite interface locations as shown which moderates the flux fringing effect which helps keep the temperature rise more under control.
Just to mention, I once saw someone try to make a fifty watt flyback power supply with just a center post gap alone. The first prototype lasted about five minutes before the flyback transformer started emitting smoke.