A Parallax is when the scene is moving, that objects in the background move slower compared to objects closer to the camera when it is moving.
I made my own Parallax scene in photoshop to see how i could make one.
I created a gradient with colours for the sky and the ground for my main background which won’t move. Then, I added some clouds in the sky. I then added a few more layers; which included mountains, hills and city buildings made by using the paint tool.
After this, i then decided to make it an animation. I added 10 frames and moved the mountains across 2 pixels per frame, the hills 4 pixels per frame and the city buildings by 8 pixels per frame. This made it so the mountains in the back of the scene moved slower than the hills/buildings.