A sub-menu is a derivative menu that is reliant on a top-level menu in a graphical user interface system. Common software environments such as Microsoft Windows have more or less trained users to be able to recognize controls for accessing sub-menus, which are often invisible from a start screen.
Techopedia explains Sub-Menu
The use of a sub-menu involves a strategy of nesting one menu inside another. For instance, under a main menu titled "tools" or "options," there may be a sub-menu with a title like "format" or "insert," which has its own list of options as well.
One of the most common examples of sub-menus is in Microsoft Windows, where many of these sub-menus are accessed by clicking on a top-level menu with the mouse, dragging the mouse down to a sub-menu row and moving the mouse, usually to the right, toward what is called a sub-menu arrow. As the mouse moves off of the main menu, in the vicinity of the sub-menu arrow, the sub-menu appears. Although this has been largely an effective interface for users, some have problems controlling the mouse in a way that opens the sub-menu, because of the relatively small window for error. Developers also have to code these operations carefully, sometimes with visually mapped sub-menu access routines, in order to prevent bugs or errors.