90+10 means that those staying at a Disney resort may call 90 days from their check in day and make dining reservations for the first day of their vacation + the next 10 days.
If you are not staying at a Disney resort you can call 90 days from your first day of vacation and make an ADR for your first day only. You then have to call back the next day and can make an ADR for your second day and so on.
I hope that makes sense.