Introversion is a personality trait characterized by being reserved and less social. Introverts are more easily overwelmed by socializing over longer periods and need time alone to "recharge the batteries".
The opposite of being introverted, is to be extroverted. Extroverts can be much more social, and often enjoys it more, they are often fond of partying and mingling. Everyone possesses both traits to a greater or lesser degree.
The video shows Howard Stern and David Letterman talking about close friendships, it may sound as though both are introverts(!). Watch the entire broadcast/interview on the late night show's website.