SpaceX has been working to make its rockets partially reusable since as early as 2011. Up until now, practically all orbital rockets have been expendable, so they’re basically thrown away once they launch into space. That means an entirely new rocket — which can cost tens to hundreds of millions of dollars to make — has to be built for each mission to orbit.
SpaceX’s strategy has been to land its rockets after launch in an effort to fly them again and again. That way the company can partially save on manufacturing costs for each mission.