If you are social, you like to be around people. A social butterfly is someone who is social or friendly with everyone, flitting from person to person, the way a butterfly might.
The word social comes from the Latin socius meaning "friend." When you're being social, you're everyone's friend. Go to a social, or mixer, and you might make a lot of new friends. Stand in the corner pouting, however, and you're being anti-social. Humans are described as social beings — ones that tend to move or live in groups together, like penguins!