A one time only opportunity to present yourself to people who haven't had the chance to meet or know you. You will get a bad first impression, or a good first impression from someone, and they will get either one from you, unless you come across as neutral. The moment of first impression is your limited window of opportunity to ensure others get the right feel for you and your personality.
Often though you will meet people for the first time in conflict, which means showing the positive side of yourself is impossible. A second chance is hard to come by, not always solving the negative first impression, but can help the first impression if you show humility.
Good first impression
If you can manage this, people will appreciate your company more, and you may be on the way to making some actual friends. On a date, this may mean you will get lucky, or at least the adoration of the person your with. Ultimately, this success is what is sold in advertising, since most people feel insecure about their natural abilities to get good first impressions. The sagest advice i've been given is that if you be yourself, you will be 100 times more successful in being seen a great person than if you pretend to be something your not.
Appearance is not all that is involved in a good first impression. If you can be witty, sometimes even silly within limits, your personality will round out any kind of negative features you may have physically. This is what saves the human race, and gets people laid online.
Bad first impression
A number of things can lead to a bad first impression, mostly things not commonly held as positive social appearance. If you go on a first date with a girl (or boy), and you have snot dangling from your nostril.. the person will not be able to focus on your personality. Of course it is not possible to know what other people are thinking, they may like that. However the law of averages dictate that most people prefer those who are hygenic conscious.
Another way to achieve a bad first impression is by fighting with the person. It is much easier to insult and be defamatory towards someone you do not know, this also has a name people like to call burning bridges. You often do not want to convey yourself in such a manner, since a bad first impression may spread to all the people that person knows and affect your reputation as a whole.
So don't be a punk (unless you actually are a punk, in which case take these definitions as polar), and go for the good first impression instead.