Yeah, you unlock refrain after doing the routes, In addition to that you unlock 3 more routes after refrain.

What MOW said. Refrain is unlocked after some routes is completed. This game also includes Ex which can be unlocked after completing Refrain.

As the above said.

Just to clarify, the 3 extra routes (they are full stories) known as "Ex" (as said by BadHacker above) was released as a later game in Japan (Little busters EX) and this is why they aren't feature in the original play through.
The new routes can be triggered by a choice which is added near the start, you start a new game as normal and to keep it spoiler free a new event happens one of the first days relating to a notebook, this will open the other routes (i can't remember the choices on how they all work as its been some time).