Parallax is the apparent shift in the position of an object when you look at it from different viewpoints. It’s a depth cue our brains use to understand how far away things are. In web and UI design, parallax usually refers to an effect where the foreground content and the background move at different speeds as you scroll, creating an illusion of depth and a subtle 3D feel to the page. Designers use it to make pages feel more immersive, guide the eye through a story-like layout, and add visual flair without real 3D.