Quickie definition:
Snippet: a little bit of PHP code that does something like getting the document IDs and names from the database, making a menu out of them, and the results get displayed on the page in the same place you called the snippet. All you have to do is style the results with CSS.
You tell it exactly what you want it to do by giving it arguments:
?startDoc (or &startDoc if it’s not the first one, just like hyperlinks and query strings) is a variable (inside it’s called $startDoc, so PHP knows it’s a variable), since it could hold any document ID.
The beauty of a CMS is that you really don’t need to learn a lot; the snippets will be "black boxes" that you just plug into your HTML wherever you want them. You do need to learn what arguments they take, and what they will do if you don’t give them any arguments. Like DropMenu will always use 0 as its $startDoc if you don’t give it one. So if you want it to use the root of the site as the base to build the menu on, you can just skip giving it one, it will use 0 by default.