I was looking at the membership benefits and was wondering what is Priority Check-In?

You've earned more than points. You've earned better living on the road. As a member, you'll receive the recognition you deserve at more than 2,700 hotels worldwide, plus free nights, free flights and many other benefits, including:

1- Priority check-in
2- A newsletter, including member-only specials
3- Bonuses and base points earned at all Marriott hotel brands