A binary option broker specialises in binary options, and brings supply and demand for these options together. So if you want

to buy a binary option on the S&P 500 (a U.S. stock market index) the broker looks for someone who wants to sell this option.

Because there is no market where binary options are sold directly, if you want to make a trade, you must always do this

through a broker.

More info:
http://www.binxc.com/binary-option-brokers