overflow : hidden is a property which will make any text going out of your div as hidden i.e. it will not be shown on screen and will be clipped. overflow : auto will make scroll bar's appear if the text goes out of your div.

Why do we use overflow hidden in navigation bar?

Adding overflow: hidden; triggers a new block formatting context that prevents . navBar from "collapsing" when it has floated children. Some people will suggest using display: inline-block; . Use with caution as each element will have white space around it that will make them larger than you think.

What is overflow used for?

The overflow property specifies whether to clip the content or to add scrollbars when the content of an element is too big to fit in the specified area.