Okay kiddo, so imagine you have a special toy clock that helps you keep track of time. This toy clock has a pendulum that swings back and forth, and every time it swings back and forth it makes a "tick-tock" sound.
Now, a crystal oscillator is kind of like that toy clock, but instead of a pendulum it uses something called a crystal. This crystal vibrates very quickly, thousands or even millions of times per second, instead of swinging like a pendulum.
The frequency of a crystal oscillator is basically how many times the crystal vibrates each second. It's like how many "tick-tocks" the toy clock makes in a minute.
Different crystals can vibrate at different frequencies, just like some toy clocks might tick-tock faster or slower than others.
We use crystal oscillators in lots of different electronic devices, like computers and phones, to help them keep track of time or generate precise signals. And by choosing a specific crystal with a specific frequency, we can make sure that these devices work correctly and stay synchronized with other devices.