sealing wire after you solder them... comes in different sizes based on the wire gauge, different colors etc. Run a match or heat gun over it and it shrinks down and makes a nice covering on the connection. I use a bunch of it doing stereo installs and working on my truck since I don't like crimp on connectors.
It also comes in bigger sizes and is used for a water tight seal on splices in underground electric lines. They also make a "cold " shrink as well.
I am assuming you are asking this in regard to archery. It is used primary on the prongs / launcher arm of arrow rests, so when a bow is drawn the arrow pulls silently on the tubing rather than on the metal or plastic of the prongs / launcher arm of the rest.
