Options/Display/Layer (see chapter 7.10.1.26) allows to set-up that there will always be drawn a frame around certain layer types. This way one can see at a glance which layers are existing and are selectable.
But sometimes it is more important to get a visual overview of a document and in this case these frames are quite disturbing. Therefore Show Layer Frames allows to quickly turn on and off these frames.
Double-clicking the icon
opens the display settings of layers.