The web is built on links, and web users routinely use links to find the information they need. When a site links to your site, web users follow those links, resulting in web traffic, or referral traffic. When these backlinks, also called inbound links, come from high quality sites, they also tell Google your content is authoritative and trustworthy. This is important because inbound links from respected sites are an SEO ranking factor, so they can boost organic traffic, too.
Websites produce traffic rankings and statistics based on those people who access the sites while using their toolbars and other means of online measurements. The difficulty with this is that it does not look at the complete traffic picture for a site. Large sites usually hire the services of companies such as the Nielsen NetRatings or Quantcast, but their reports are available only by subscription.
Not all web traffic is welcomed. Some companies offer advertising schemes that, in return for increased web traffic (visitors), pay for screen space on the site. There is also "fake traffic", which is bot traffic generated by a third party. This type of traffic can damage a website's reputation, its visibility on Google, and overall domain authority.