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:
