The Step property allows changing the step through what the Series Labels will be shown. By default, the Step property is set to 0, so Series Labels will be shown on each Series. The picture below shows a chart with the Step property of Series Labels set to 0:
If the Step property is set to 2, then Series Labels will be shown as it is shown on picture below:
The value 1 of the Step property indicates that Series Labels will be shown for each value of Series.