Reputation on Steemit is essentially a gauge of quality of an individual's posts and an indicator of how long they've been around. Someone with high reputation is typically more respected and usually has more followers than someone with a low reputation. Also when it comes to flags, if someone with a higher reputation than you flags you, your reputation goes down. If someone with a lower reputation than you flags you, your reputation is untouched. In short it's just a metric to keep an eye on quality. Spammers, people that plagiarize, and mean people usually have low reps. Community leaders, those that post great content, and those that are trusted and respected usually have high reps.