It’s the name given to menus that instead of having a complete menubar, put all the items in a menu that is open by pressing a single button. Usually the button icon is three horizontal lines, supposed to look like a menu with 3 items, but people have nichnamed it “hamburger” as it also looks vaguely like a stylized bread-meat-bread stack.
This type of menu appeared on phones where it made sense to save screenspace and have rather square-ish shaped buttons, but now ofr some reason it spreads to Desktop systems, where it is a bit nonsensical because there is a lot more space so we don’t need to hide menus this way, and our pointing devices are more precise, so items don’t need to be relatively large squares.