The z-index is a CSS property that determines the stack order of an HTML element. An element with with the greater z-index value will be in front of the element with lower values. It is a very useful property once you fully understand and it fixes a lot of layout issues.

Read the “The Z-Index CSS Property: A Comprehensive Look” article at Smashing Magazine to learn the ins and outs of the z-index and start using it on your next web development job.