What is tip and ring?

Tip and ring are really holdover terms from the beginning days of telephone service, when there was an actual tip and an actual ring on a telephone jack. During those times, telephone operators used plugs or jacks to connect customer calls on a switchboard. Tip and ring were the wires connected to the different parts of the plug itself, one side being connected to the metal tip of the plug, and the other side being connected to a metal ring behind that tip.

Source: Blog.T3innovation.com