The top reviewer list is determined by who writes the most reviews - and the most helpful reviews, as judged by other IFDB members.
Top Reviewer status is a ranking of who has the most Frequent Fiction points. The Top 10 Reviewers are the ten members with the most points, and likewise for the Top 25, Top 50, and Top 100.
IFDB is a community enterprise. There's no elite group of appointed experts who get to decide which games are good and which aren't. Everyone can contribute, and Frequent Fiction points are meant to provide some small recognition for that.
Note that Top Reviewer status is dynamic, because the status is always relative to other members. You and others can earn more points any time. The site calculates your points and keeps them up to date automatically, so you don't have to do anything special to make sure your points are counted.