On Alarm Clock Plus, what does "Allow auto snooze/dismiss" mean? How do I use these features and can they both be used on one alarm?
Snooze - Instead of shutting the alarm off, it would pause it allowing you to sleep for few more minutes. If you see your settings, you could notice the snooze minutes viz - 5, 10, 15 minutes and you can also keep custom minutes. 
When the alarm rings and you feel like having a quick 5 min extended sleep, you hit the snooze option. This comes handy since you dont need to wakeup, set another alarm for next 5-10 minute and doze off. 

Dismiss - This cancels/shuts the alarm down for that day &amp; time.
You click on the dismiss option when you are ready to ware up! 

Auto Snooze
That means if there isn't any action (if you don't hit the snooze options) system automatically snooze and again the alarm rings after the x minutes as per the snooze interval. If this auto snooze is not set, you would probably get a notification as "missed alarm". I just updated the answer aswell. 

Hope that helps!