The post, part of a series titled "How CSS Works," explores the concept of z-index in CSS, a property that controls the stacking order of elements on a webpage's two-dimensional display. It elucidates that z-index can manipulate the paint order of elements, making them appear "closer" or "farther" from the user, but this only applies to elements with a position other than static and within their respective stacking context. The article explains that stacking contexts can be created using various CSS properties, such as position values or opacity, and highlights the potential confusion arising when elements display in unexpected layers due to local stacking contexts. By understanding these nuances, developers can apply z-index more predictably in multi-layered UIs, avoiding common pitfalls and ensuring the desired display order of elements.