Patrollers are users who have the ability to mark new articles as having been patrolled. The purpose of patrolling new articles is to ensure that they are not vandalism or disruptive.
How do you become a patroller?[edit source]
There is no pre-determined process to become a patroller. Since this flag only lets people mark edits as patrolled and allow all their own edits to be automatically patrolled, any user can contact a sysop and request the flag. Please note that you actually need to be a member of the community and have made some contributions: the flag will not be granted to users who don't have decent contributions or are not making enough new pages.
A list of patrollers can be found at here.