Phonology is the system of abstract sounds that are used to encode the language. This doesn't refer to the actual pronunciation details of the language, nor to all possible sounds the language can produce.
Instead, a phonology of a certain language consists only of the simple (non-compound) sounds of that languages which are meaningfully distinguished from other sounds. These sounds, called phonemes, are the most basic signs of the language, and they are used as the building blocks of all the more complex signs (such as words and entire sentences). For example, the phoneme /h/ in Japanese sounds differently depending on which vowel follows it. To English speakers (who have a different phonology than Japanese speakers), it would sound like their own /h/ phoneme before the vowel /a/, but like their /f/ phoneme before the vowel /u/. For Japanese speakers, however, this is the same phoneme, that just happens to have two different pronunciations.
for questions discussing actual pronunciation details, such as how to make a Japanese /r/ or what are the different realizations of the Japanese /h/ phoneme.
- Japanese phonology on English Wikipedia