It means the offense doesn't put the defense in bad positions and scores points, controls the ball to minimize other team's opportunities and doesn't turn the ball over.
The defense gets stops and turnovers and does so without giving up a bunch of yards so that punts end up pinning them inside the 10, which then gives the offense a bad drive start.
Special teams limits opposing returns, pins them deep and makes plays when we have the ball as well as making kicks when we need them to.
Essentially, all 3 areas of the game play well and more importantly play together in a way that maximizes the chance to win THAT game. That's the key point. On a game to game basis, what that means is situational.