Usually front desk staff ask for the last four of the credit card on file or other simple question and you are on your way to the room. Lately (in North Carolina only), they ask for everything. Last night I checked in at the Renaissance South Park in Charlotte NC and I was asked for credit card on file, ID card and AAA card (I booked a AAA rate). After showing everything requested I asked why: the answer was for the hotel security.
I looked on the Marriott website but I could not find a detailed explanation of what it means except that you check in using the app and then stop at the front desk to just pick up the key and be on the way to your room. Which leads me to believe that some hotels do not follow procedure. If they insist on asking for everything, they shouldn't use Mobile Check In.
So, what does Mobile Check In mean?