In my opinion, the title The Force Awakens refers to the awakening of two characters.
We are introduced to Rey in her humble and meager life of poor scrounging, as she patiently awaits the return of her family to Jakku. Throughout the film, she is pure, she is skilled, and she is strong. As she faces Kylo Ren and his clearly strong grasp of the Dark Side, something awakens (unintentional word choice) inside her, and prevents Ren from accessing the information he needs. Slowly, she gains her powers (a point Ren himself acknowledges). Over time, she becomes stronger in her recognition of the Force within her, culminating in one of the final scenes of the movie where her newfound strength, originating from the Force, saves her. Also: she is always dressed in light colors, and her outfits look typically Jedi.
Throughout much of the film, Luke feels like a J.J. Abrams nod a what Hitchcock called a macguffin: something that drives plot but never really appears on screen. Of course, this only applies to the first… two hours and fifteen minutes of the film or so. Early on, we learn that Luke abandoned his teachings when Kylo Ren was seduced by the Dark Side and disappeared. Our heroes must fight the First Order without him for the entirety of The Force Awakens. Many die. But when the mission is complete, and Rey leaves the rebel base, she heads to the location on the now completed map to Luke. Upon finding him atop a mountain(?), Rey offers Luke his old lightsaber. Perhaps Rey will be the catalyst that causes Luke to finally return.
Most likely: it is a combination of the Force “awakening” in Rey, and the Force in Luke returning to the rebels.