A black tie event is a highly formal event (such as a dinner, wedding, or other social gathering) where a tuxedo should be worn by the men (not just any suit). In the U.S. we call it a tuxedo, in Britain and other English speaking countries it is called a dinner jacket. Women should wear a cocktail dress or evening gown. http://en.wikipedia.org/wiki/Black_tie

It means that the guys have to wear dinner suits and bow ties and the women have to wear after 5 evening wear. This could refer to an important reception, ball, wedding or other important affair.

It is not an informal meeting. That's why dinner jackets should be worn (= the event is black tie). Dinner jackets are black or white jackets worn with a bow tie at formal occasions in the evening.

To attend a black-tie event, you need to wear suits and of course black tie ^^