A session is a group of interactions a user has with a website during a defined period. Google Analytics starts one when the user visits and ends it after 30 minutes without interaction, though site owners can adjust that timeout. A new session also begins if the date changes while the user remains on the site, or if the campaign that brought the user changes and they return through another campaign.
One person can generate several sessions in a single day without ever really leaving the brand behind, browsing in the morning, getting distracted, coming back that evening still counts as two separate sessions in Analytics even though it was one ongoing shopping decision from the visitor's own perspective. That's worth remembering when reading session counts as a proxy for unique visitors; the two numbers measure genuinely different things, and a high session count doesn't necessarily mean a large or growing audience, just a lot of distinct visits, however they're actually distributed across real people.
