六千万 is a number meaning '60 million' (literally 'six thousand ten-thousands'). In Chinese, large numbers are grouped by ten-thousands (万), not thousands, so 六千万 is the standard way to say 60,000,000. It is used in contexts like population, money, or statistics. As a measure word, it is a numeral-classifier compound that can directly modify nouns without an additional classifier (e.g., 六千万人 '60 million people').