A phoneme is the smallest contrastive segment in a language. In other words, they are the smallest building blocks that make the difference between two different words.
If one swaps the /ppp/ in pin to /bbb/, the word becomes bin This changes the meaning from a pointy metal sewing tool (pin) to a box (bin). Phonemes are the building blocks that allow us to build words.
Phonemes by themselves do not have meaning. The phoneme /bbb/ does not mean anything.