Harassment is the act of repeatedly annoying or pestering someone.
Harassment is especially to be avoided when clubbing. A term sometimes used is "Energy Vampire": someone who, just by the way they watch or behave, detracts from the 'positive energy' of the atmosphere at a club.
The worst thing to do is look at people as if they are zoo exhibits. At a zoo you can laugh at the monkeys. You can talk loudly about computers. You can say the baboon is ugly, because you don't care what the baboon thinks about you. If the baboon looks at you, you don't need to acknowledge its attention. It isn't important.
Treating people like that is wrong. Don't do it.