This is usually metal or plexiglass that is placed over the keyboard. Cutouts in the keyguard allow you to increase your accuracy. Keyguards are available for every type of standard keyboard. Many choices are available in alternative keyboards. Both of these are membrane keyboards with large surfaces.
Membrane keyboards have flat surfaces with keys defined visually or physically by a keyguard.
With the right software, both can be programmed to meet your specific needs that might not be matched by the commercial overlays. If a keyboard is not an option for you, but you can use a mouse or can point, an onscreen keyboard could be a solution. An onscreen keyboard is software that presents the image of a keyboard on the screen. You access this with a mouse, trackball, joystick, switch, touchscreen or electronic pointing device. You click on the letter or character desired just as if you were using a regular keyboard and it is accepted by your word processor or whatever application you are currently using.