Tintamarre skeleton/theme

I am studying the Tintamarre skeleton and I have one question regarding partials/navigation.html.twig. The designer makes references to menu items using uppercase followed by the t filter: {{ "MENU_HOME"|t }}. I could not find any reference in Google , the grav or Twig documentations. Can someone tell me the following:

1. Where is MENU_HOME declared or defined? This question also applies to the other menu items.
2. What does the "t" filter do? Not referenced in Twig doc.

Thank you in advance.

Thank you Olevik. Your answer was spot on