Z-index is a CSS property that allows to specify the stack order of specific elements. An element with greater stack order is always in front of another element with lower stack order.
Here is how you can use Z-index in CSS:
```
.element {
position: absolute;
z-index: 1;
}
```
In the above example, the .element has a z-index of 1. This means that it will appear on top of any other element with a lower z-index, or no specified z-index.
Here are a few important notes to keep in mind when using z-index:
1. Z-index only works on positioned elements (i.e., those with position: absolute, position: relative, or position: fixed).
1. Z-indices can be positive or negative. An element with a negative z-index will appear behind an element with a z-index of 0 or auto.
1. If two competing elements have the same z-index, the one that appears last in the HTML will show on top.
1. Z-index can create a stacking context, meaning it can control how elements stack together as a group. For example, if an element with a z-index of 1 contains child elements with z-index of 1000, those child elements will still appear beneath an element with a z-index of 2.
