When almost everyone farmed for a living, that was an agriculture economy (or agrarian, if you’re being fancy). When everyone works in factories, that’s an industrial economy.
A knowledge economy has a large number of participants involved in knowledge creation, or in the leveraging of knowledge for economic advantage. So, lots of scientists, programmers, inventors, mathematicians, etc.
Basically, the knowledge economy is what you think of when you think of Silicon Valley...the kindly, loving, American-blue-collar-worker people at Facebook, Apple, Google, etc.