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 we take word like pin and change the first phoneme to /b/, we change the word to bin This changes the meaning from a pointy metal sewing tool (pin) to a box (bin). Phonemes function as building blocks and allow us to build words.

Phonemes by themselves do not have meaning. The phoneme /b/ does not mean anything.