Normally, if a layer is checked in the Contents window, ArcGIS Explorer will draw it. However, as you zoom out, it may become difficult to see the more detailed information, or as you zoom in, information may become too coarse. While you can turn off a layer, this may be inconvenient, especially if your map contains several layers or if you change the scale frequently as you work.
The ability to set the scale range for a layer's visibility is useful because you can progressively display more detailed layers as you zoom in on an area. If a layer isn't drawing because it has a visible scale range set, you'll see the layer name dimmed in the Contents window and if you move the mouse over the layer name, an explanation will appear:

To set the minimum scale at which a layer is visible
- Select the layer; then, on the Appearance tab, in the Display Range group, choose the a scale by name in the Minimum Scale dropdown control or click the Use Current button to set the Minimum Scale to the scale the map display shows currently.. When you zoom out further from the map than this, the layer will not be visible.

To set the maximum scale at which a layer is visible
- Select the layer; then, on the Appearance tab, in the Display Range group, choose the a scale by name in the Maximum Scale dropdown control or click the Use Current button to set the Maximum Scale to the scale the map display shows currently. When you zoom in closer to the map than this, the layer will not be visible.
To clear the scale range for a layer
- Select the layer; then, on the Appearance tab, in the Display Range group, click Clear.