In parking lot parlance, the verb "to key" means to deliberately scrape another person's car. In the context of construction, keying is the process of roughening a surface by scoring or sanding it, to provide a receptive surface for the application of paint, plaster, rendering, tiles, etc. It's often used between one layer of plaster and the next—the more surface area each new layer can adhere to, the stronger the bond.
Depending on the project, keying can be done with a professional scratching tool, rough sandpaper, or a board with drills poking out a millimeter or two on one side (AKA a "devil float"). If you're using such an improvised too, don't press too hard―the goal is to seal the layers together without leaving marks on the final surface.
Use a figure eight motion to cover as much of the wall as you can, with a loose goal of leaving no gaps wider than six inches between any scored lines.
A keyed surface may also refer to the spaces between laths or wire meshes, which provide a grip for plaster.