That's the day your FP falls on. Your check in day is 60, the next day is 60+1, the following 60+2, etc. So for me, my days went to 60+4. I have 5 days we're spending in Disney. I didn't book any FPs on our +4 day. I booked them all 60 to 60+3. Some rides are easier to get later in your trip. I had to put Mine Train on my 60+3 day. You don't have to specifically know that when booking FPs though. You just pick the day in the park and start looking for your FPs.