The simple definition: For real estate agents and brokers in the US, IDX is the system that allows REALTORS® to show MLS property listings on their websites. IDX stands for "Internet Data Exchange", which sounds technical, but really it's quite simple from the agent's perspective.
For example, if you are a real estate agent, you might have an MLS property search on your website. This feature is made possible by an IDX "data feed" from your MLS board.
Professional real estate website designers like AgentWebsite® provide website packages that include IDX. This makes it easy for agents and brokers to show all mls properties on their own website. In fact, it's a great way to generate leads.