Finishing panels are non-drilled fronts that can be used to create sides, create filler or even customised elements, such as niches or shelves, to harmonise your layout.
They should be screwed from the inside of the cabinet. 

Side panel

Side panels are finishing panels that allow you to hide the outside ends of your cabinet. They are not necessary if you are installing your unit between 2 walls, but they are essential for units placed at the end of a row (high and low units), for the side of columns, the underside of high units and the back of central islands.

Filler

A filler is a finishing panel that fills the gap between a wall and a cabinet, and is fitted flush with the fronts.

Leg panel

A leg panel is a finishing panel that supports the countertop and connects it to the floor. It must be provided if you are designing your kitchen with a dishwasher or washing machine at the end of the countertop: as the appliances are lower than the cabinet, a leg must be added to support the countertop. 

Here is an article that might interest you: What is a finishing panel? 💡