What Is The Drop Down Menu And How Would I Use It?

It is not clear that which context you are asking about drop down menu.
But simply in context of window based applications we can define the drop down menu as 'a list of options which are not seen unless its title is clicked by the user'.
In other words in a software the drop down menu is hidden list of commands/options (such as open, save, cut, copy etc.)  With a visible title and that list becomes visible when the user clicks on that title (the name of menu).

After the visibility of that list the user can move up and down among those options/commands by dragging the mouse or by arrow keys of keyboard. The user can select one of those commands to be performed.

When the focus is out of that list that list of options (menu) disappears.
Normally a large number of options that are relevant are kept under one heading or title. For example the commonly seen 'edit menu'keeps the options (like cut, copy, paste etc.) that are used for editing of data on a document. Thus drop down menu are used to save space on a window and can provide more than one options/commands with one title.