Visits are the data used to drive Gatekeeper. Typically, visits are synonymous with web page requests. That is, each time a user loads a page, a visit for that request is recorded. The visit object contains information about the request, such who the visitor is and what they are attempting to do.
Additionally, a visit can be authorized, whereupon Gatekeeper will make a recommendation on how to handle the request. Visit authorization is a core feature of Gatekeeper. See the visit authorization documentation for more information on how visits are authorized.