Sideplating is the metalization of the board edge. Edge plating, Border plated, plated contour, side metal, these words can also be used to describe the same concept.

Sideplating is used to shield electrical noise radiating between layers, extra cooling, board to board connections, etc. 

If you want to specify edge plating on your design, run your copper layer beyond the board edge by a minimum of 500um. Ensure there is a minimum of 300um inside the board edge and leave a 800um gap between the board edge and non-conducting layer.

Use the soldermask layer to expose the copper edge design and include text to communicate any detailed requirements.

Immersion gold or silver are the two most common options.