A broker is a place where buyers and sellers go to buy and sell instruments, such as currencies.
The forex broker operates as a middleman between you and the market. In other words, in order to find a buyer or a seller of currencies, you can go to a broker and they match you up with either a respective seller or a respective buyer.
However, instead of just being the middleman between you and another buyer or seller, they are also the middlemen between you and what is called a “liquidity provider”.