It means that when someone thinks about another person, they usually access the first memory in their data banks of that person. Which would be their first impression of that person. =]
Sure it means that when a person first meets someone if you are not your usual self that person will always remember that first time.
Sometimes when a person done something stupid or genius it all depends on the other person's perception and how they received this perception on that person will leave a lasting impression on you.
It means that as humans we tend to size people up in 5 minutes or less based on our past experiences. So when you first see a person, you size them up and there is nothing to do to change that moment in another persons mind.
It means when you first meat someone for the first time don't be rude don't dress in riped torn and stained cloths