S-turns are ground-reference maneuvers designed to improve your flying skills when confronted with wind, which is most of the time. If there were no wind for this maneuver you would simply fly half a circle on one side of a road or other marker, then level out, turn, and fly the other half of the circle. The trick is compensating for the wind.
As with other ground-reference maneuvers, the key is compensating for the direction and strength of the wind. To compensate for direction, remember to crab or turn the nose slightly toward the wind. To compensate for wind strength, remember that the wind will increase your ground speed when flying away from it and slow you down as you fly toward the wind.