Phonology

It makes use of the phonetics in order to see how sounds or signs are arranged in a system for each language. In phonology, it matters whether sounds are contrastive or not, that is, whether substituting one sound for another gives a different, or "contrastive," meaning. 
For example in English, [r] and [l] are two different sounds - and the words "road" and "load" differ according to which of these sounds is used. But in some languages, [r] and [l] are variations of the same sound. They could never make a meaning difference in words that differ by only that sound.

Phonologists describe the contrastive consonants and vowels in a language, and how pronunciation is affected by the position of the sound in the word and the sounds that are nearby. They are also interested in syllables, phrases, rhythm, tone, and intonation.

There is this video that explains about what phonology is.

This link will direct you to the explanation about what phonology is. 

Phonology: The Sound of Pattern Language