Kapton tape (made from polyimide) is an insulation tape. It has a high resistance to both heat and electricity.
Kapton tape makes an excellent surface material for 3D printing with a heated bed.
After a piece has been printed from a 3D printer it begins to cool and as it cools the plastic starts to shrink. If the piece does not cool evenly this shrinking can warp the piece by, for example, causing the corners or ends to curl upwards.
A heated bed helps to ensure that pieces cool evenly and thus prevents them from warping.
To use Kapton tape as a surface material for a heated bed it must be applied, without bubbles, to a sheet of glass.
Kapton tape makes an ideal heated bed surface because it bonds well with hot, recently printed plastic but once the pieces are cooled they can also be easily peeled off the Kapton tape without causing any damage to the piece or the tape.