To paraphrase Debbie's description. Phase 1 is about nailing the basics time after time with some vision stuff thrown in.
A small tight 30 or so second course with most of the elements you will encounter is setup with each element/segment being mostly timed separately so you can see where you need to focus, and can see how one element affects the next. The instructors drive your car at times to provide examples, and insight, and ride with you most of the time.
Segment times are really useful, and I think with the exception of the Chicago box which I think I only did well once or twice, I was faster than both instructors in my car both Pase1, and Phase 3.
I would recommend Phase 1 to anyone who has done at least 1 autocross, and we had a guy or two who had never done any. Even veterans of 100s of autocross runs will learn, including me...
And you have to have had Phase 1 to take Phase 2