Is a control element placed in the top corner of the user interface that visually looks like 3 horizontal lines and from a bird's-eye view looks like a nice burger.
"The first big app that used a hamburger button was Path, I believe. Then it was popularised by Facebook, and the hamburger button started to appear from app to app and many apps started to use them.The biggest one is you can put a lot of information inside it, and at the same time provide a consistent way to find this extra information. Facebook is a great example of this." — Alex Tyagulsky, Readdle
"What’s out of sight, is out of mind."
That little three-lined button is the devil. Whether you call it a side menu, navigation drawer, or a hamburger, hiding your features off-screen behind a nondescript icon in the corner is usually a poor mobile design choice. Interaction theory, A/B tests, and the evolution of some of the top apps in the world all support the same thesis: the hamburger button is bad for engagement, and you should probably replace it with a tab bar or other navigation scheme.