ARP - Advance Reservation Period
"Advance Reservation Period" is the period during which you may buy a reservation ticket for a particular train in Indian Railway. This ARP duration varies for different types of trains. Most of them follows the same ARP which could be 3 to 4 months, i.e 90 days or 120 days. But, some trains have a different ARP like 30 days or even 15 days. Indian Railway rarely changes this ARP period.
An ARP of 120 days means, a reservation ticket could be booked 120 days in advance of the date of journey from the source station.
During 'online ticket booking' through IRCTC, you might encounter a message "outside ARP period". This means that the reservation for that date has not been started yet, for that particular train.