This is how it was explained to me: All 3 rings require some movement for the most part, even the Stand ring. The Stand ring requires that you stand up and sort of move around for a minute every hour...it is not perfect, you sometimes can get credit for standing and not walking around, but for the most part it requires that you stand up and walk around or stretch for a minute. The Move ring requires that you move around a bit more and raise your heart rate somewhat. This would be good for most leisure or mild exercise activity. As far as the Exercise ring, think of it as minutes you maintain your heart rate at a certain elevated level. If your heart rate drops below that elevated level while you are exercising, you will still get Move credits.
I go on 2 walks everyday. The first walk is a 35 minute walk with a senior citizen and I get no exercise credit (or sometimes 1 or 2 minutes of credit) for it even though I enter it in the workout app, but it does give me Move credit. Later in the day, I go on a 70 minute very brisk walk and I usually get about 60 to 65 minutes of Exercise credit from that to complete my exercise ring.