On a computer program or form, a "drop-down menu" is a small box which expands to show more choices when you click or tap it.
Most computer programs have drop-down menus at the top of the screen which contain different actions and settings that you can select. Online forms may have drop-downs for questions with a limited number of answers. For example, the "Country" option on a form is often a drop-down menu.
You can use the phrase "drop-down menu" or just "drop-down".