Web APIs (navigator, geolocation, history)

What browser feature is this code designed to handle?
window.addEventListener('pageshow', function(event) {
  if (event.persisted) {
    console.log('This page was restored from the bfcache');
    refreshPageContent();
  }
});

window.addEventListener('pagehide', function(event) {
  if (event.persisted) {
    // The page might enter the bfcache
    savePageState();
  }
});
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