CSS positioning refers to the techniques used to control the layout and positioning of elements on a web page. There are several positioning methods available in CSS, including static, relative, absolute, and fixed positioning. Additionally, the z-index property is used to control the stacking order of positioned elements. Let's explore each of these concepts:

1. Static Positioning: By default, all HTML elements have static positioning. In static positioning, elements are positioned according to the normal flow of the document. They are not affected by the top, bottom, left, or right properties, and the z-index property has no effect on them. Static positioning is the default behavior and doesn't require any additional CSS properties to be set.

2. Relative Positioning: Relative positioning allows you to move an element relative to its normal position in the document flow. When an element is set to position: relative, you can use the top, bottom, left, and right properties to offset the element from its original position. Other elements on the page will still respect the original space occupied by the relatively positioned element, even if it appears to have moved visually. The z-index property can also be applied to relatively positioned elements to control their stacking order relative to other elements.

3. Absolute Positioning: Absolute positioning allows you to position an element relative to its closest positioned ancestor or the entire document itself if no positioned ancestor exists. When an element is set to position: absolute, you can use the top, bottom, left, and right properties to position it precisely. Absolute positioning takes the element out of the normal flow of the document, and other elements will ignore the space occupied by the absolutely positioned element. The z-index property can be used to control the stacking order of absolutely positioned elements.

4. Fixed Positioning: Fixed positioning is similar to absolute positioning, but the element is positioned relative to the browser window rather than its ancestor. When an element is set to position: fixed, it remains fixed in its position even when the page is scrolled. This is commonly used for elements like headers or navigation bars that should stay visible at all times. The z-index property can also be applied to fixed positioned elements to control their stacking order.

5. z-index: The z-index property is used to control the stacking order of positioned elements. It takes a numeric value, where higher values place an element on top of elements with lower values. Elements with a higher z-index appear visually in front of elements with a lower z-index. The z-index property only has an effect on elements that have been given a position value of relative, absolute, or fixed.

It's important to note that when elements overlap, the stacking order determined by the z-index property can impact which element is visually displayed on top.