Reputation is an indication of how much the community trusts you; it is earned by convincing your peers that you know what you're talking about. Basic use of the site, including posting reviews, does not require any reputation at all. But the more reputation you earn, the more privileges you gain.
The primary way to gain reputation is by posting good content (reviews, images, etc.). Votes on this content adjust your reputation. The Leaderboard shows the top contributors.
All users start with one reputation point, and reputation can never drop below 1.
At the high end of this reputation spectrum there is little difference between users with high reputation and moderators. That is intentional. We don't run this site. The community does.