The selected layers can be made invisible by choosing the "Hide" option on the "Visibility" panel. Choosing "Show" makes the layers visible again.
Also, double-clicking a visible layer in the layer list toggles the layer's visibility.
To make a layer "transparent" (i.e. let the other layers show through), select "Transp." on the "Visibility" panel. To make it opaque again, select "Opaque" (the default).