The summary is meant offer a quick overview of your site's statistics.
The "General information" table
Shows a summary of the site. Things like date of registration, visitors record, etc.
The first graph
Shows you site's hits in real time.The boxes at the bottom stand for the last 15 seconds. THe numbers in the boxes are the number of hits you site has had in that particualr second. The green second is the current seccond.
The second graph
Shows the evolution of visitors, visits, and hits in the last 12 hours.
The "Traffic information" table
Will show you the number of visitors, visits, and hits for today, yesterday, the last 24 hours, the last hour, the last 7 days, the last 30 days, and the total for your site since registration on xxxStatistics.com.
Visitors
The number of distinct people that have visited the monitored site. A visitor can have more visits.
Visits
Each visits represents a succesion of consecutive hits from a single visitor. A visit ends when the visitor closes the browser or leaves the monitored site for good.
Hits
The number of times a page from your site was loaded in the visitor's browser.