What's the meaning of "unplugged"?

This usage of unplugged probably originates with the long-running TV show MTV Unplugged, in which well known musical artists perform their hits using acoustic instruments rather than electrically powered ones--for example, playing traditional guitars instead of electric guitars, or a piano instead of a keyboard. By extension, therefore, "[subject] unplugged" is sometimes used colloquially to indicate that the subject is being presented in a more low-key and intimate context then that in which he or she is usually seen.

As JanusBahsJacquet notes in the comments:

Electric instruments have to be plugged in to function properly. If an instrument is unplugged, it won't work—unless it's an old-fashioned, acoustic instrument. Therefore, concerts that are played with acoustic (and usually minimal) instruments are called unplugged concerts, often in the form Lady Gaga unplugged (for example). Extending that to non-concert uses, it means something like the person being unprepared, without a safety net, taking things as they go along. If a politician gives an unprepared speech, for example, he can be called ‘unplugged’.

Unplugged

The OS X dictionary says:

unplugged |ənˈpləgd| adjective 1. trademark (of pop or rock music) performed or recorded with acoustic rather than electrically amplified instruments.

Meaning you still have microphones and amplifiers and speakers. But are not using instruments that have a cord directly plugged into them. Even though there are accoustic guitars and violins etc. that have built in microphones and a cord to attach an amplifier to them.