The toolbox and the controls you can drag
The toolbox is a catalogue of ready made controls. Dragging one onto a page drops a line of markup and an object you can program against.
How the list is grouped
Controls are grouped by purpose: standard controls such as buttons and labels, validation controls, data controls, and so on. You rarely need more than a handful at first.
Dragging compared with typing
Dragging is fine while you are learning the names. Once you know them, typing the markup is often faster and always more precise. Both produce exactly the same page.
What a control really is. Each control becomes an object on the server with properties you can read and change in code. That is why a control has both a visible face and a programmable one. The ASP.NET from scratch track shows this in practice.
After a control is on the page, you shape it in the properties panel.