As far as I know, phonology studies sounds of a particular language. So, what does the writer mean by 'across language'?
"Phonology describes the way sounds function within a given language or across languages to encode meaning."
Here's an example: The /b/ in English differs from the /b/ phoneme in French; the English /b/ is followed by a puff of air called 'aspiration'. That is why the Berlitz 'sounds like' tables (at the beginning of their phrasebooks) have a very limited use (limited, that is, to the needs of minimal touristic use).