What is the yellow component shown here? This is off a Outlet USB phone charger.
That is a transformer, used to convert one voltage to another.
As others have remarked, the component is a transformer.
I see capacitors on both sides of the transfer, so my first guess is that this circuit rectifies the AC input, followed by an SMPS which uses the transformer to crate the lower voltage.
As a first-order approximation the amount of power that a transformer can transfer per cycle is determined by the magnetic flux that its metal core can contain. Hence the same transformer can transfer much more energy when used at 50 KHz than when used at 50/60 Hz. This could explain why such a small transformer can transfer an amount of energy that would require a much bigger transformer when used at 50/60 Hz.
But again, this is my guess based on almost no information ;)
It is a step down transformer to reduce the AC outlet voltage to a lower level before rectifying it. This is done so the rectifier can be made with more reasonable parts (smaller diodes etc). Using a 10:1 transformer means you're now dealing with 12Vrms versus 120Vrms!
In a high quality phone charger the output will then be fed into some sort of regulator to ensure the output stays constant regardless of AC mains spikes.
its a transformer, it converts AC voltages, it either steps it up or down, on a USB charger its a step down. It steps down 110VAC or/to 220VAC to a voltage level the other power conversion components can safely handle.