Parallax scrolling is when things on your computer screen, phone, or tablet move at different speeds when you scroll up or down. It's like when you look out the window of a moving car and things closer to you seem to move faster than things further away.
Imagine you are watching a movie where a car is driving down the road. When the camera is close to the car, you can see all the details of the car and the road it's driving on. But when the camera moves further away, the car looks smaller and the road looks longer. This is like parallax scrolling on your screen, where different parts of the screen are moving at different speeds.
So, when you scroll down a page and see the background moving slowly while the text or pictures move faster, that's parallax scrolling! It can make the page more dynamic and interesting to look at, just like watching a cool movie scene.