a phoneme is the smallest unit of sound that has meaning in language. for example, the s in books in english shows that something is plural , so the sound /s/ has meaning. different language use a different rang of sounds and not all of them have meaning in other languages. for example, the distinction between /s/ and /sh/ is an important one in english, where it helps distinguish between words such as so and show, sock and shock , sore and shore. but in cantonese, you can use either /s/ or /sh/ in words without changing their meaning, i.e. in cantanese they are not two separate phonemes.