If you’re not familiar with the term, a cookie is a way to store information about your browser session, and persist that data over a period of time.
Before cookies were created, websites had no idea who you were from one page view to the next. Compared to the pervasive tracking of today, that sounds like a dream come true.
But it also meant that websites had no way to log you and keep you logged in, or remember preferences from one page to the next.