The /b/ sound is a bilabial sound. Bilabial sounds are made by the lips pressing together. Try saying “bat” and notice how your lips press together initially to form the sound. Minimal pairs are groups of words that differ in one phonological element and /p/ and /b/ are minimal pairs. The difference between the two sounds is that /p/ is a softer sound with a little less airflow. It is voiceless while the /b/ sound is voiced, forcing more air through the vocal mechanism and vibrating the vocal chords.
Voiceless means that you do not use your voice to say the sound as in the /p/ sound, and /b/ is a voiced sound because you do turn your voice box on to make the sound. If you put your hand on your throat and make the /p/ sound, there is no vibration but if you do the same thing while making a /b/ sound, you feel a vibration. Also, if you put your hand in front of your mouth and say the /p/ sound, you will feel a puff of air, whereas, there will be almost no air with the /b/ sound.
Voiceless means that you do not use your voice to say the sound as in the /p/ sound, and /b/ is a voiced sound because you do turn your voice box on to make the sound. If you put your hand on your throat and make the /p/ sound, there is no vibration but if you do the same thing while making a /b/ sound, you feel a vibration. Also, if you put your hand in front of your mouth and say the /p/ sound, you will feel a puff of air, whereas, there will be almost no air with the /b/ sound.