I've discussed elsewhere that usability is about real people doing real tasks in a reasonable amount of time. Some researchers also refer to "learnability" and "memorability" to define usability—this is very similar to discoverability. Can you discover the features of the system just by poking at it? Is the user interface obvious enough that you can figure it out on your own?
Daniel's post includes a video of a usability test that explores the Google search box on a smartphone. It's a short but very interesting video to watch, as the tester clearly becomes more and more frustrated with how to initiate a search.
I want to close by highlighting some of Daniel's comments:
if there is no way to discover what operations are possible just by looking at the screen and the interaction is numbed with no feedback by the devices, what's left? the interaction gets reduced to experience and familiarity where we only rely on readily transferred, existing skills.
And that's why usability is so important, especially for open source software where users often must learn the software with no tutorials or other instructions to guide them.