A session is a group of user interactions that take place on your website within a given time frame. This can be a user landing on your website, visiting multiple pages (each individual pageviews), submitting a form (an event) and then purchasing a product (an eCommerce event). Each of those interactions was individual but together made up one single session from a user.
By default, a session lasts until 30 minutes of inactivity in Google Analytics, but that can be adjusted without your own settings. This means that if your user starts a session and interacts with your site but then gets up and leaves their desk for 30 minutes or doesn’t return to the tab for 30 minutes, that session will count as closed. If they return after that, it will be treated as an entirely new session.