Active screen edges allow you to activate effects by pushing your mouse cursor against the edge of the screen. Here you can configure which effect will get activated on each edge and corner of the screen.
Click with any mouse button onto a square and select an effect in the context menu. Edges with a black square have already an attached effect, a grey colored square indicates that no effect is selected for this edge.
The number of accessible items in the context menu depends on the settings in the module Desktop Effects in the Workspace Appearance and Behavior category. Select Enable desktop effects to activate compositing, select the compositing type OpenGL, and then choose your favorite effects on the All Effects tab from the Windows Management group. This activates the corresponding items in the context menu.
If you are looking for the setting to enable switching of desktops by pushing your mouse cursor against the edge of the screen choose one of the Present Windows effects from the context menu.