Basically it was what happend between Episodes IV and V, but with the new characters

Lets compare:

At the end of ANH, the Rebels destroy the Death Star, but the Imperial Army still drives them from their base, and persues them across the Galaxy. At some point Luke has a run in with Vader, where Vader learns who he is. Finally they settle on a base on an ice planet until they are driven off of that and spend the rest of the time in space.

The Resistance destroys Starkiller Base, but the First Order still drives them from their base. They then are chased across the Galaxy, taking heavy losses. Rey then has a confrontation with Kylo Ren, and the Resistance ends up on a base on an ice planet, until their driven off of that and spend the rest of the movie on a ship.

So instead of writing a new story, they just took the events that happened off screen inbetween the first two movies and made the movie based off of that.