Click on the vertical TOC bar on the right edge of the map or shift-click on map to display Map Layer Selection.

The shows all the layers that are available in this map service and allows you to manage which are visible.

Click on to open or close a group and reveal or hide layers in the group.
Click on to reveal classes.
Click on to turn off labels.
Click on to turn on a group, layer, or class.

says the layer is on, but not visible due to the scale of the map.
is displayed if only some of the classes or layers are on.