It means you can board the train and hope for an empty seat in the non-reserved cars. If there is no empty seat, you can stand until you reach your destination.

Great option if:

• You entered the station just moments before the train departs and you don't have time to reserve a seat
• The reserved cars are sold out and you have no choice
• You're only going a short distance and you don't mind standing in the space between the cars
• You're boarding at the first station, and you're willing to get in line earlier than other people with the same idea **
• You're sure that there are few people traveling and there's bound to be an empty seat

The last two items in the list require more than a visitor's sense of how things play out in Japan, Since the price of a reserved seat is not a significant saving, a lot of people reflexively purchase reserved-seat tickets.

** Be aware that the other people with the same idea are willing to get in line earlier than you.