A more thorough explanation:

A government-securities interdealer broker is a type of broker that exclusively deals with government securities for parties who are themselves government brokers or dealers. A broker is an agent who acts as an intermediary or negotiator between prospective buyers and sellers. They are employed to make bargains and contracts between other persons in matters of trade, commerce, or navigation.

For example, a government-securities interdealer broker may facilitate the buying and selling of U.S. Treasury bonds between two government brokers or dealers. They do not deal with individual investors or retail clients.

This type of broker is important in the financial industry because it helps to facilitate the trading of government securities between large institutions. This can help to increase liquidity in the market and ensure that government securities are priced efficiently.