When you place the cursor on >fdef or >defn – which Cursive doesn't know what to do with out of the box – a light bulb appears right next to it. This is IntelliJ's quickfix feature for errors it can easily correct. You click it, select 'Resolve as...' and then 'defn'. I think the default shortcut ist Alt+Enter.
Basically, the macro system makes it possible to extend the language in userspace via libraries. This allows the core language to stay small and focused, while different ideas can be tried out as usage patterns change.
The way Clojure is used today is very different from the way it was used a decade ago. However, the language itself has stay remarkably focused over time.
EDIT: I'll update the docs to make this clearer.