What Does Keycap Mean?
A keycap is a small plastic cover placed over a key on a standard computer keyboard. Each keycap is marked with the letter or function the system performs when the respective key is pressed. The standard material used for keycaps is plastic and the most common color is black, although they are also available in white, gray and other colors.
Techopedia Explains Keycap
Early keyboards were manufactured with keycaps and key switches as one unit. Later, with the introduction of overlay keyboards, the keycap became a separate component so that it could be placed in the user's desired location. In order to increase their usability, blank keycaps are also available with no pre-printed label. This is especially useful for overlay keyboards. The lettering on keycaps is usually done either by stamping or by laser for durability.
Custom keycaps have become a popular computer modification, with keycaps available in a wide variety of colors and styles.