How to see JavaDoc in IntelliJ IDEA?

Go to File/Settings, Editor, click on General.

Scroll down, then ✔ Show quick documentation on mouse move.

Alternatively you can position your cursor on the item and show JavaDoc using

CTRL+Q

which is the default shortcut.

Edit: As Methical mentioned on Mac the shortcut is

CTRL+j (^+j not ⌘+j)

Use View | Quick Documentation or the corresponding keyboard shortcut (by default: Ctrl+Q on Windows/Linux and Ctrl+J on macOS or F1 in the recent IDE versions). See the documentation for more information.

It's also possible to enable automatic JavaDoc popup on explicit (invoked by a shortcut) code completion in Settings | Editor | General | Code completion (Autopopup documentation):

Yet another way to see the quick doc is on mouse move: