A phoneme is the smallest contrastive sound unit in a language.
A phoneme is usually represented by a single letter or symbol in a language, however it can be pronounced in a number of ways without changing the meaning.
For example the phoneme /s/ in English can be used to form plurals.
While it always spelt with an /s/ it is not always pronounced the same way, eg, compare the final sound in dogs and cats.
The /s/ after the word dog is actually pronounced like a [z] sound while after cat it has the more typical [s].