A hamburger menu is a graphical element often used as a button in a graphical user interface, typically an icon consisting of three parallel horizontal lines (displayed as ☰), which is used to trigger a hidden menu. It’s called a hamburger menu because of its similarity to a hamburger.
In web design, hamburger menus are used to hide navigation items in a compact form, typically on smaller screens such as mobile devices where screen real estate is limited. This allows to the preservation of the page design and simplifies the navigation, making it more accessible and user-friendly on smaller screens.