A field-effect transistor (FET) is like a little gate that controls the flow of electricity. Imagine you have a toy car and a gate. If the gate is open, the car can drive through. If the gate is closed, the car can't drive through.
Now, imagine the car is electricity and the gate is made of a special material called a semiconductor. When you put electricity into the gate, it creates an electrical field that either turns the gate on or off. When the gate is on, electricity is allowed to flow through the FET. When the gate is off, the FET blocks the flow of electricity.
FETs come in different shapes and sizes, but they all work the same way. They are used in all sorts of electronic devices, like phones, computers, and even cars! FETs are important because they are very efficient at controlling the flow of electricity, which makes electronics work better and use less power.