In this context, "visitors" refers to visitors to your website.
Visitors are usually considered as an aggregate count reported by log analysis software or on-page tracking (e.g. Google Analytics). Most systems distinguish between "all visitors" and "unique visitors". A visitor might visit your site multiple times during a time period, and the latter attempts to count such cases as a single visitor.
Ideally visitor analysis will also distinguish (and filter out) spiders, robots, and other automated systems.