Reputation by definition is the beliefs or opinions that are generally held about someone or something. On steemit the reputation score is built with an algorythm.
The idea is to reward good quality content and to protect from bots abusing the steemit system. Still algorythms are vulnerable to exploits, even the reputation algorythm. It gives you an indication where to find quality content and which one you even might avoid.
As mentioned the reputation score is built on an algorythm and could be changed anytime.
It is not linear, it works on a logarythmic scale. A reputation score of 60 is 10x more effective than a reputation of 50, 100x more than a reputation of 40,.
New accounts start with a score of 25 . If for some reason you get downvoted(flagged) below zero, your posts will be hidden from public.