A phoneme is the smallest unit of sound that has meaning in a language. For example,the "s" in books in English shows that something is plural,so the sound /s/ has meaning. Different languages use a different range of sound and not all 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 Cantonese they are not two separate phonemes.