Kapton is a trademark of DuPont.
It refers to a polyimide film used in many applications. A common application is dielectric tapes used to insulate electrical connections.
In DCC, it is useful to secure decoders. It is very thin, yet strong. Kapton tape with silicone adhesive can used to attach decoders and wires to the locomotive frame. The adhesive used does not tend to ooze out over time, like cheap vinyl tape does. It is stronger and cleaner than the typical vinyl electrical tape sold in many stores. Being thinner also means less problems with clearances and binding. Kapton does not withstand mechanical wear very well.
It is used extensively in the semiconductor and electronics industry due to it's high temperature tolerances, especially for masking circuit boards during wave soldering operations.
Compared to black vinyl electrical tape, it is very expensive.