Hello there! A Gunn diode is a device that helps create microwaves. When you turn on the device, it creates a special type of electric field, which makes something called "domain walls" inside the diode start moving. These walls are kind of like tiny traffic jams inside the diode.
As these walls move, the electrons inside the diode start getting pushed and pulled in different directions. This creates a little hill that the electrons have to climb over. When they get to the top of the hill, they suddenly get a lot of energy and start vibrating very quickly. This vibration is what creates the microwaves that the Gunn diode is used for.
So, to sum it up: a Gunn diode is a little device that helps create microwaves by making electrons move around in a special way. It's kind of like shaking up a soda can to make bubbles appear!