This will probably be moved to meta
In short, users who are active on other sites in the stack exchange network gets an association bonus of 100 reputation points on this site. The requirement is to have above 200 reputation points on an SE site.
The reason is that such users probably know the format, and some of the restrictions for new users, such as commenting, doesn't make sense for users that knows the format.
In addition, 100 is not high reputation. It essentially bypasses some checks placed on new users, such as voting, commenting on other peoples questions, participate in chat etc.
High reputation is a couple of thousand and upwards, where you get privileges such as edit questions and answers without approval, vote to close and so forth.