If you can explain what a USB PID, a J state, a K state, and an SOF are, you can probably stop reading now. But if you don’t know or you want a refresher, you can spend 15 minutes watching [Sine Lab’s] straightforward explanation of the USB protocol details. You can find the video below.
The motivation for this is he wants to add USB to his projects using an ATMega with a hardware USB implementation. Honestly, most of the time, you’ll just consume some premade library and get it working that way. However, understanding the terminology can help you, especially if things don’t go as planned.