what's an Aheadset

The headset is a pair of bearings which allow the fork to turn smoothly. If you look at the bit of a bike there the fork enters the frame there's a little round gubbins at the top and bottom of that bit of the frame. These are the two parts of the headset. If you look at the picture you posted it shows these two parts sat one on top of the other. They each look like a mushroom (the bottom one an upside down mushroom), where the thin part is normally hidden inside the frame.
The reason it's called an Aheadset is because the way they worked changed quite a few years back and they called the new design an aheadset to distinguish it.

thanks for the reply. now how do you take the headset off? mine is not an aheadset because the steerer tube is only 1" (not 1 1/8"😉.

It depends what sort of headset you have. The size of the steerer tube is not what differentiates the two designs.
A "normal" headset uses a steerer tube which has a thread cut into it, and which only just sticks out of the top of the frame. These headsets use two great big nuts above the top part of the headset which screw onto the steerer tube to hold it all together. Then the stem has a wedge which sits inside the steerer tube to hold it in place.
An aheadset uses a non-threaded steerer tube which sticks a couple of inches out above the frame. In this case the headset is just slotted into the frame and the stem tightens down onto the steerer tube to hold it all in place. This is preferred these days because it's lighter and doesn't need huge spanners to tighten it up.

As for fitting headsets, the usualy recommendation is to find a nice shop to do it, it's tricky because they usually fit very tight and fitting or removing them normally involves the use of a mallet!