"The second girl is British."
Translation:A doua fată este britanică.
It modifies the first word. In this case, the numeral is before the noun, so it has a definite article, while the noun has no article (ordinal numerals appear before the noun).
A better example to illustrate this is with a regular adjective, which usually comes after the noun, but can also be placed before it:
ideea mare / marea idee - the big idea