It means the number of discrete times an individual gets on the site and spends time, multiplied by everyone who uses the site. Thus, if you get on the site in the morning and again in the evening, that's two website sessions. If you're like me and you get on the site every time you get the chance, you might have three or four in a day! :)
Website sessions are visits to the site by individuals - sessions time-out after a period of inactivity on the site. So, around 710,000 times per month, individuals come and visit the site. Some maybe once, some daily, some several times per day.