The popstate event is triggered during history navigation: 1) It fires when the active history entry changes due to user navigation with back/forward buttons, 2) It also fires when navigation occurs via history.go(), history.back(), or history.forward() methods, 3) The event object contains a state property with the state data previously passed to pushState() or replaceState(), 4) Importantly, popstate does NOT fire when history.pushState() or history.replaceState() are called, 5) The event is essential for single-page applications to detect browser navigation and update content accordingly, 6) It allows web applications to synchronize their UI with the browser's navigation state, 7) The event bubbles and is not cancelable, 8) Without handling this event, back/forward navigation in single-page applications would change the URL but not update the displayed content.