Sometimes what's on PCB is just too confusing and there's benefit to seeing only the stuff on the layer you're routing on. If you press Shift + S you'll cycle in/out of Single Layer Mode; a mode in which only the currently selected layer is visible and all other objects are masked. When you change layers in this mode, it'll follow you to the subsequent layer. Nice for tidying up text on the PCB too! If you want to control what layers are being displayed, use the 'L' shortcut to launch the Board Layers and Colors dialog and enable/disable layers. See images below.