When searching around or keycaps, the term "double shot" frequently appears. These kinds of keycaps use a unique manufacturing process in which two pieces of plastic get molded together.
Typically, the main housing of the keycap is one piece, while the cap's legend is separate. This double shot method ensures that the legends never fade over time since they are separate pieces of molded plastic.
The other method for putting legends on keycaps is to print them on. However, these printed legends run the risk of fading over time.
As you use the keyboard and your fingers constantly rub against the surface, it is only a matter of time before they chip off. Double shot keycaps will stay sharp, consistent, and never rub off.