Yes, a modern online defensive driving course is built to work on a phone or tablet just as well as a computer. With Fast Track, there's no app to install and nothing to configure, you register, log in through your browser, and the course adjusts automatically to whatever screen you're using.
Not much. The lessons, quizzes, and audio option all work the same way they do on a desktop. The main difference is convenience, you can work through a lesson on a break, in a waiting room, or anywhere else you have a few spare minutes, instead of needing to sit at a computer.
A stable Wi-Fi or cellular connection helps avoid interruptions mid-lesson, and using the free audio option is especially convenient on mobile, you can listen while doing something else instead of needing to stare at a screen the whole time.
Taking the course on your own schedule, on whatever device is in front of you, is part of what makes it one of the more convenient options available. It pairs naturally with the audio and read-along option if you'd rather listen than read on a small screen.