A session is a period of related activity. On a website, it often means one visit: you open the site, move between pages, and then leave. Some websites use a session to remember temporary details, such as what is in your shopping basket while you browse.
Think of it as a visit that starts when you arrive and ends later. The exact rules depend on the website or technology. A browser tab's session storage, for example, can keep data while that tab remains open, but a website session is not always the same thing as a browser tab.