An IX is the internet, or at least a small part of it. It's sort of a network where each device shares their "routes" with the others, and then propagates those

An IX is the internet, or at least a small part of it. It's sort of a network where each device shares their "routes" with the others, and then propagates those routes back through their own network. They're exchanging their respective pieces of the Internet with each other. The devices themselves are just routers, though much larger and more complex than what you'd see in a house or office. Instead of one route, "put all the traffic through eth0", they'll have hundreds, thousands, or millions of routes depending on their location relative to the "rest of the internet".