Heat-shrink tubing has been around for quite a while, and is an easy way to finish and insulate an electrical connection. It is used in place of electrical tape — which is also fine to use — because it is neater and easier, especially in small applications.
To make a connection, slide one piece of tubing over one of the two wires that are to be joined. Twist-connect the bare wires. Fold the twisted wire together and back along itself; slide the tubing over the connection. Pass the tubing over a heat source (such as a lighter or candle flame) and it will instantly shrink to protect the connection. Note: the tubing shrinks in length as well as diameter so position it carefully over the connection.