Mt. is the written short form of Mount, put in front of a mountain's name: Mt. Fuji, Mt. Everest. You write the short form; you still say 'Mount' when you read it out loud. In American writing the full stop is common (Mt.); British writing sometimes drops it (Mt). It is for names, not for the common noun — you do not write 'a mt.' when you mean 'a mountain'. Maps, road signs, postcards, and addresses use it to save space. Though actually, the same letters can look like other short forms (Mountain in some street names, and in US writing Mt can also be Montana), so the name after it is what tells you this is the mountain sense. If you see 玉山 in English as Mt. Jade, that is this use.